diff --git a/.claude/skills/feature/SKILL.md b/.claude/skills/feature/SKILL.md
index e03ed2d21..84abf9d20 100644
--- a/.claude/skills/feature/SKILL.md
+++ b/.claude/skills/feature/SKILL.md
@@ -98,7 +98,7 @@ Continue to scope analysis below.
 ### 2. Module boundaries
 
 - All new code isolated inside the target module (`modules/{module}/`)
-- No modifications to shared core files (`lib/middlewares/`, `config/`) — except `config/templates/` for new email templates; additions to `lib/helpers/` or `lib/services/` require explicit justification
+- No modifications to shared core files (`lib/middlewares/`, `config/`) — except `config/templates/` for a new Devkit-owned email template (this repo); a downstream project's own new template goes to `config/templates/<project>/` with a `mailer.templates` map entry instead, never the flat folder (see `lib/helpers/mailer/README.md`). Additions to `lib/helpers/` or `lib/services/` require explicit justification
 - Module registers its own capabilities via exports (policies, subjects) and file discovery (config auto-discovered by filepath pattern) — auto-discovered by the core
 
 ### 3. Analyze flows & edge cases
@@ -174,7 +174,7 @@ A service whose every function is a 1:1 forward to the repository (no branching,
 If an action affects another user:
 - Use `lib/helpers/mailer/` abstraction (never nodemailer directly)
 - Check `mailer.isConfigured()` — skip silently if not configured
-- Create template in `config/templates/` for each new email type
+- Create the template: `config/templates/` for a Devkit-owned module (this repo), or `config/templates/<project>/` plus a `mailer.templates` map entry for a downstream project (see `lib/helpers/mailer/README.md`)
 - Send async, non-blocking (`.catch(() => {})`)
 
 ## Phase 2 — Definition of Done
@@ -196,7 +196,7 @@ If an action affects another user:
 - [ ] Grep existing enum values to find all locations before committing
 
 **Module autonomy:**
-- [ ] No shared-file changes unless explicitly required; any `lib/helpers/` or `lib/services/` additions include explicit justification, and `config/templates/` is used for new email templates
+- [ ] No shared-file changes unless explicitly required; any `lib/helpers/` or `lib/services/` additions include explicit justification, and a new email template goes to `config/templates/` (Devkit module authors) or `config/templates/<project>/` + a `mailer.templates` entry (downstream projects)
 - [ ] Module self-registers capabilities (subjects, abilities) via policy exports
 - [ ] New routes/middleware defined inside the module boundary
 
diff --git a/MIGRATIONS.md b/MIGRATIONS.md
index 0a7f21d75..78184fde0 100644
--- a/MIGRATIONS.md
+++ b/MIGRATIONS.md
@@ -22,6 +22,17 @@ Set `config.mailer.brand.signature` (and `brand.logoUrl`/`brand.primaryColor`/et
 
 ---
 
+## mailer: downstream-only templates need no migration; expect every shared mail's rendered shape to change (2026-10-04)
+
+#4134, rounding out the #4133 body-fragment migration above:
+
+- **A template that only ever existed in your project** — added via a `config.mailer.templates` entry, or resolved through the flat `config/templates/<key>.html` fallback — needs no migration at all. Devkit's ISO merge only overwrites a filename it ships, and none of the 15 converted mails are new filenames a downstream could collide with.
+- **Expect the rendered HTML of every shared mail to look different**, even with no config change on your side: each now renders inside the shared layout's own `<head>`/outer table and brand-driven header/footer instead of its old standalone markup. A visual or snapshot diff on a shared mail is the expected result of #4133 above, not a regression.
+
+See `lib/helpers/mailer/README.md` for the full customization guide (brand, layout/partials, templates, and the namespace rule).
+
+---
+
 ## skills: thin service re-export, zero-logic unit-test exception, `create-module` `crud-only` option (2026-10-03)
 
 `.claude/skills/{naming,feature,create-module,verify}/SKILL.md` updated — no runtime code changed:
diff --git a/config/defaults/myproject.config.js b/config/defaults/myproject.config.js
index f7479b8b8..36303050c 100644
--- a/config/defaults/myproject.config.js
+++ b/config/defaults/myproject.config.js
@@ -1,7 +1,7 @@
 /**
  * Downstream project config template.
  *
- * 1. Copy this file and rename it to config.{yourproject}.js
+ * 1. Copy this file and rename it to {yourproject}.config.js
  * 2. Set NODE_ENV={yourproject} when running the app
  * 3. Override only the keys you need — development defaults are used for everything else
  *
diff --git a/lib/helpers/mailer/README.md b/lib/helpers/mailer/README.md
new file mode 100644
index 000000000..42c7e47dc
--- /dev/null
+++ b/lib/helpers/mailer/README.md
@@ -0,0 +1,221 @@
+# Mailer customization guide
+
+`lib/helpers/mailer/index.js` renders a handlebars template to HTML and sends
+it through a provider (`nodemailer` or `resend`, picked by
+`config.mailer.provider`). Every shared Devkit mail — and any mail a
+downstream project adds — goes through the same three independent
+customization levels, from broadest to narrowest:
+
+1. **`mailer.brand`** — the values every mail shares (name, logo, colors, a
+   sign-off). No template to touch.
+2. **`mailer.layout` / `mailer.partials`** — the HTML shell every mail body
+   renders inside, and its four building blocks (header, footer, button,
+   styles).
+3. **`mailer.templates`** — a single mail's body.
+
+A fourth mode, **legacy full-document passthrough**, lets a project ship a
+mail that is already a complete HTML document: it skips the `mailer.layout`/
+`mailer.partials` shell (Level 2) — no automatic header/footer/button — while
+still receiving `brand`, `appName`, and `appContact` through `render()`'s
+params, for the template to use itself if it wants them.
+
+Read `lib/helpers/mailer/brand.js`, `layout.js`, `paths.js` and `index.js`
+for the implementation — this guide follows what they actually do.
+
+## Namespace rule
+
+Devkit owns every file directly under `config/templates/` (the 15 shared
+mails) and every `config/templates/_*/` folder (`_layout/` today — the
+default layout + its partials). **A downstream project's own templates —
+new ones, or a customized copy of a shared one — always go under
+`config/templates/<project>/`**, reached through a `mailer.templates` entry
+(Level 3 below), never dropped straight into the flat folder.
+
+Two reasons this matters, not just style:
+
+- `render()` sanitizes a template **key** to its `path.basename()` before
+  resolving it — so you can't reach a subfolder by renaming the call site
+  (`template: 'myproject/welcome'` still resolves to a file named
+  `myproject/welcome`, which `path.basename` reduces to `welcome`). The only
+  way to point a key at a file outside the flat folder is a `mailer.templates`
+  map entry, whose *value* (not the key) carries the real path.
+- `config/templates/*.html` is `/update-stack`'s ISO-merge scope, the same as
+  `modules/` and `lib/` — a file you add straight into the flat folder is
+  fine until Devkit ships a file with that same name, at which point the next
+  ISO merge treats it as Devkit's and overwrites it.
+
+## Level 1 — `mailer.brand`
+
+`getBrand()` (`brand.js`) resolves one brand object, merged into every
+render. Three fields fall back to existing app config so the legacy
+`appName`/`appContact` params resolve to the same values as before
+`mailer.brand` existed — but the shared mails' rendered HTML did change (new
+layout shell, new footer; see `MIGRATIONS.md`) even with an unconfigured
+brand. The rest have no fallback — they render only when set:
+
+| Field | Fallback | Rendered by |
+|---|---|---|
+| `name` | `config.app.title` | header (as text, or `alt` on the logo), and the legacy `appName` param (below) |
+| `url` | `config.app.url`, then `getBaseUrl()` | footer, as a plain link |
+| `contact` | `config.app.contact` | footer, as a `mailto:` link, and the legacy `appContact` param (below) |
+| `logoUrl` | — | header (replaces the text name when set) |
+| `primaryColor` | — | the button partial's background, and the footer's link color |
+| `textColor` | — | the layout's email-shell text color |
+| `fontFamily` | — | the layout's email-shell font |
+| `signature` | — | footer, first line (the old per-template sign-off) |
+| `footerText` | — | footer, second line |
+| `mutedColor` | — | not used by any shipped partial — reserved for a project that ships its own `mailer.layout`/`mailer.partials` (Level 2) |
+| `links` | `{}` | footer, one link per `{ label: url }` entry |
+
+```js
+// config/defaults/<project>.config.js — same shape as
+// lib/helpers/mailer/tests/mailer.brand.unit.tests.js
+export default {
+  mailer: {
+    brand: {
+      name: 'Rocket',
+      url: 'https://rocket.example',
+      contact: 'support@rocket.example',
+      logoUrl: 'https://rocket.example/logo.png',
+      primaryColor: '#112233',
+      textColor: '#ffffff',
+      mutedColor: '#999999',
+      fontFamily: 'Helvetica, sans-serif',
+      signature: 'The Rocket Team',
+      footerText: 'Rocket Inc.',
+      links: { Pricing: 'https://rocket.example/pricing' },
+    },
+  },
+};
+```
+
+Every scalar field is also settable individually via
+`DEVKIT_NODE_mailer_brand_<field>` (e.g. `DEVKIT_NODE_mailer_brand_primaryColor`),
+same as any other config key. `links` is an object, not a scalar — one env
+var can't set it; an env var always delivers a string, which fails
+`getBrand()`'s plain-object check and falls back to `{}` (configure `links`
+from a `config/defaults/<project>.config.js` file instead).
+
+**Legacy `appName`/`appContact`:** every render also receives `appName` (=
+`brand.name`) and `appContact` (= `brand.contact`) as plain params, for
+templates written before `brand` existed. Brand wins over a caller-passed
+`appName`/`appContact`/`brand` — those three keys only; every other param
+the caller passes through `mail.params` reaches the template untouched.
+
+## Level 2 — `mailer.layout` and `mailer.partials`
+
+Any template that isn't a full HTML document (see Level 4) is a **body
+fragment**: it's compiled on its own, then wrapped inside `mailer.layout`
+(default `config/templates/_layout/layout.html`), which also pulls in four
+named partials (default `config/templates/_layout/{header,footer,button,styles}.html`):
+
+```js
+export default {
+  mailer: {
+    layout: 'config/templates/myproject/layout.html', // falls back to Devkit's own when unset or ''
+    partials: {
+      header: 'config/templates/myproject/header.html', // override one, or several — the rest keep Devkit's default
+    },
+  },
+};
+```
+
+Both paths go through the same containment check (`paths.js`): relative,
+`.html`, and resolved inside the project root — an absolute path, a `../`
+escape, or any other extension throws, naming the offending config key.
+
+The layout's own render context is **not** the template's: it receives
+exactly `body` (the compiled fragment, the only unescaped slot), `brand`,
+`subject` (fills `<title>`), and an explicit allow-list of two link params —
+`emailSettingsUrl`, `unsubscribeUrl` — lifted from the caller's `params`.
+Nothing else from `mail.params` reaches the layout or its partials; a footer
+that wants a third, per-mail/per-recipient link needs that key added to the
+allow-list in `index.js` — a Devkit stack change (`lib/` is ISO-merge scope,
+so a downstream edit there is overwritten on the next `/update-stack`), not
+something a downstream project edits directly. A downstream wanting extra
+footer links that are the same on every mail adds them through `brand.links`
+(Level 1) instead, one `{ label: url }` entry at a time.
+
+The `button` partial takes its own hash args from the invoking template,
+not the layout context — the pattern every shared template uses:
+
+```handlebars
+{{> button url=url label="Get started"}}
+```
+
+Overriding `mailer.partials.button` without matching this call shape (a
+`url` and a `label`) breaks every template that invokes it.
+
+## Level 3 — `mailer.templates`
+
+```js
+export default {
+  mailer: {
+    templates: {
+      // key → project-owned relative .html path, same containment rule as Level 2
+      welcome: 'config/templates/myproject/welcome.html',
+    },
+  },
+};
+```
+
+Resolution for a given key (`resolveTemplatePath()` in `index.js`): a
+`mailer.templates` entry wins; anything else falls back to
+`config/templates/<key>.html` — today's behavior, so a template that only
+ever existed in a downstream project (never one of the 15 shared mails)
+keeps resolving with no config at all. That flat-fallback path is what
+customizing a **shared** template in place would silently rely on too,
+which is exactly what the namespace rule above says not to do.
+
+## Level 4 — legacy full-document passthrough
+
+A template whose source starts with `<!doctype` or `<html` (case-insensitive,
+leading whitespace allowed) is detected as a complete document and sent
+**unwrapped** — no layout, no partials, no brand-driven header/footer. It's
+compiled on its own pristine handlebars instance, so it can't use
+`{{> header}}`/`{{> footer}}`/`{{> button}}` (those are only registered on
+the fragment/layout render path) — a legacy document that tries throws.
+None of the 15 shared Devkit mails use this path any more; it exists for a
+project that wants to ship its own, fully self-designed HTML email outside
+the shared layout.
+
+## Sender display name
+
+`mailer.from` (and a per-send `mail.from` override) is forwarded to the
+provider as-is — Devkit never parses it. Both providers accept the standard
+`"Name" <address>` RFC 5322 form, e.g.:
+
+```js
+mailer: { from: 'Rocket <hello@rocket.example>' },
+```
+
+(see `modules/users/crons/README.md` for the same pattern on a per-sequence
+`from` override).
+
+## Product-email params for a custom layout/partials
+
+A product email sent through `sendProductMail`/`announce` carries two link
+params into the layout context — `emailSettingsUrl` (the frontend's account
+page) and `unsubscribeUrl` (the one-click unsubscribe token link). The
+*default* footer partial renders only `emailSettingsUrl`, as an "Email
+settings" link, when present. `unsubscribeUrl` reaches the layout context
+too (a custom `mailer.partials.footer` may render it), but nothing renders
+it by default — it surfaces instead as the `List-Unsubscribe` header, and
+only when the resolved URL is HTTPS. A project that ships its own
+`mailer.partials.footer` (Level 2) and wants to render `unsubscribeUrl`
+itself only needs `{{unsubscribeUrl}}` in its template — it already reaches
+the layout context unconditionally (the allow-list in Level 2). For what
+each link resolves to, who gets it, and the opt-out mechanics, see
+`modules/users/doc/guides/02-email-preferences.md` — not duplicated here.
+
+## Migrating a template
+
+A project-only template (Level 3's fallback) needs no migration — Devkit's
+ISO merge never touches a filename it doesn't ship. **Converting a shared
+template you'd edited in place**, or any other breaking change to the 15
+shared mails, is dated in `MIGRATIONS.md` ("mailer: the 15 shared
+`config/templates/*.html` mails are now body fragments on the default
+layout" and the entry just below it) — including the move-to-`<project>/`
+-and-restore recipe, and that a shared mail's rendered HTML now looks
+different (new `<head>`/outer table, old sign-offs replaced by `brand.*`)
+even when nothing in your config changed.
diff --git a/lib/helpers/mailer/index.js b/lib/helpers/mailer/index.js
index 8f5de0bde..700bc2d13 100644
--- a/lib/helpers/mailer/index.js
+++ b/lib/helpers/mailer/index.js
@@ -131,8 +131,10 @@ const registerPartials = async () => {
  *   keys. Brand wins over a caller-passed `appName`/`appContact`/`brand` —
  *   those three keys only, every other caller-passed param is untouched. With no
  *   `mailer.brand` config, `brand.name`/`brand.contact` fall back to
- *   `config.app.title`/`config.app.contact` — the same values callers pass
- *   today — so output is unchanged.
+ *   `config.app.title`/`config.app.contact` — the same values the legacy
+ *   `appName`/`appContact` params resolved to before `mailer.brand` existed —
+ *   but shared mails' rendered HTML still changed (new layout shell, new
+ *   footer; see MIGRATIONS.md) even with an unconfigured brand.
  *
  *   A template whose trimmed source starts with `<!doctype` or `<html`
  *   (case-insensitive) is a legacy full document and is returned unwrapped,