docs(mailer): email customization guide, migration path, fix wrong override docs

[PierreBrisorgueil]

Summary

• What changed:
  • New lib/helpers/mailer/README.md: the mailer's 3 customization levels
    (mailer.brand, mailer.layout/mailer.partials, mailer.templates),
    the namespace rule (Devkit owns the flat config/templates/ files and
    _* folders; a downstream's own/customized templates go under
    config/templates/<project>/), legacy full-document passthrough, and
    the mailer.from "Name" <address> display-name param.
  • MIGRATIONS.md: a new entry covering the three cases — a
    downstream-only template needs nothing, editing a shared template in
    place moves it to the project folder with a map entry, and the layout
    shell change is expected.
  • config/defaults/myproject.config.js header: fixed the filename it
    names (the loader reads config/defaults/<project>.config.js, not
    what the old header said).
  • .claude/skills/feature/SKILL.md: the email-template rule now splits
    Devkit stack authors (flat folder) from downstream projects
    (config/templates/<project>/ + map entry).
  • lib/helpers/mailer/index.js render() JSDoc: corrected the
    no-brand-config fallback claim — the legacy appName/appContact
    values resolve the same as before, but shared mails' rendered HTML
    still changed with 🔧 Shared templates: convert the 15 emails to body fragments on the default layout #4133's layout, even unconfigured.
• Why: no doc told a downstream not to edit a shared template in place,
  and two docs had drifted from what the code actually does.
• Related issues: Closes 📝 Docs: email customization guide, migration path, fix the wrong override docs #4134

Scope

• Module(s) impacted: lib/helpers/mailer (docs + one JSDoc comment),
  MIGRATIONS.md, config/defaults/myproject.config.js,
  .claude/skills/feature/SKILL.md
• Cross-module impact: none (docs/comment only; no runtime code path changed)
• Risk level: low

Validation

• npm run lint
• npm test (lib/helpers/mailer/tests — 8 suites / 114 tests passed)
• Manual checks done (if applicable) — README examples cross-checked
  against lib/helpers/mailer/brand.js, layout.js, paths.js, index.js
  and the mailer unit tests

Guardrails check

• No secrets or credentials introduced (.env*, secrets/**, keys, tokens)
• No risky rename/move of core stack paths
• Changes remain merge-friendly for downstream projects
• Tests added or updated when behavior changed — docs/comment only, no
  behavior changed; existing mailer tests re-run to confirm

Notes for reviewers

• Security considerations: none — documentation and a code comment only.
• Mergeability considerations: none.
• Follow-up tasks (optional): none.

Summary by CodeRabbit

• Documentation
  • Added guidance for customizing email branding, layouts, partials, and template paths, including migration details.
  • Clarified that shared email templates now use a common layout and brand-based header and footer, so their rendered HTML may change even without configuration updates.
  • Updated setup instructions to use the correct configuration filename.
  • Clarified fallback branding when no mailer brand settings are provided.

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⚙️ Run configuration

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• Run ID: 32a6b1ec-ecd0-4c9a-ac66-f36da025cc94

📥 Commits

Reviewing files that changed from the base of the PR and between 8e9ff1c and cca3c6e.

📒 Files selected for processing (1)

• lib/helpers/mailer/README.md

Walkthrough

The changes update downstream template guidance, add a mailer customization guide and migration notes, and correct the project configuration filename instruction. The render documentation clarifies brand fallbacks and shared-mail HTML changes.

Changes

Mailer documentation

Layer / File(s)	Summary
Template boundaries and project configuration .claude/skills/feature/SKILL.md, config/defaults/myproject.config.js	Feature guidance reserves flat templates for Devkit-owned email templates and directs downstream projects to use a project-specific directory with a mailer.templates entry. The configuration template now gives the {yourproject}.config.js filename pattern.
Mailer customization and migration guidance lib/helpers/mailer/README.md, MIGRATIONS.md, lib/helpers/mailer/index.js	The new guide describes rendering modes, configuration, path rules, and related mailer behavior. Migration notes describe expected shared-mail HTML changes. The render documentation clarifies brand fallbacks and the layout shell and footer.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Other

Merge Risk: 🔵 Low · up to 8e9ff

The guide may lead downstream authors to assume full-document templates cannot use brand values, although render supplies them. Clarify the passthrough wording before merging.

Architecture Summary

Architecture risk: 🔵 Low · up to 8e9ff

The change affects 3 systems.

Changed systems: lib, config, MIGRATIONS.md

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — lib (service) was modified; 2 changed files map to changed impact.
• observed — config (service) was modified; 1 changed file maps to changed impact.
• observed — MIGRATIONS.md (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in MIGRATIONS.md: Added downstream guidance stating that project-only templates need no migration, but all shared mails render with changed HTML using the shared layout and brand-driven header/footer. It identifies visual or snapshot differences as expected and links to the customization guide.
• observed — Modified behavior in config/defaults/myproject.config.js: The copy instruction changes the target filename pattern from config.{yourproject}.js to {yourproject}.config.js.
• observed — Modified behavior in lib/helpers/mailer/README.md: Added a guide to mailer customization covering brand configuration and fallbacks, layout and partial overrides, template resolution, full-document detection, sender display names, product-email link parameters, namespace constraints, and migration guidance. It documents that configured template mappings override flat config/templates/<key>.html fallback; layout and partial paths must be relative .html files contained in the project root; full documents beginning with <!doctype or <html (case-insensitive, with leading whitespace allowed) bypass layout and partial registration; and layout context is limited to body, brand, subject, and the allow-listed emailSettingsUrl and unsubscribeUrl.
• observed — Modified behavior in lib/helpers/mailer/index.js: The render documentation replaces the claim that fallback app values leave output unchanged with a statement that shared-mail HTML still differs because of the new layout shell and footer.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the mailer documentation, migration guidance, and correction of inaccurate override documentation. It is specific to the changes, though somewhat long.
Description check	✅ Passed	The description covers the required Summary, Scope, Validation, Guardrails, and reviewer notes sections. It explains what changed and why, identifies the impacted modules and risk, reports validation,…
Linked Issues check	✅ Passed	[ #4134 ] The new mailer guide documents brand, layout/partials, template mapping, the namespace rule, full-document passthrough, and mailer.from display names. Its brand example matches the overrid…
Out of Scope Changes check	✅ Passed	All changes support [ #4134 ]. The render() JSDoc correction aligns the guide with the mailer's actual fallback and layout behavior. The additional guide details explain supported mailer customizati…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.55%. Comparing base (fe2b59e) to head (cca3c6e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4179   +/-   ##
=======================================
  Coverage   94.55%   94.55%           
=======================================
  Files         174      174           
  Lines        6134     6134           
  Branches     1979     1979           
=======================================
  Hits         5800     5800           
  Misses        271      271           
  Partials       63       63           

Flag	Coverage Δ
integration	64.05% <ø> (ø)
unit	79.85% <ø> (ø)

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[coderabbitai]

Actionable comments posted: 1

---

• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Inline comments:
Review comments at @lib/helpers/mailer/README.md:
- Around line 16-18: Update the “legacy full-document passthrough” description
to say it skips the layout and partials while still receiving brand, appName,
and appContact through render(); avoid implying that brand values are
unavailable.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: Repository: pierreb-devkit/Node/.coderabbit.yaml
• Review profile: ASSERTIVE
• Plan: Advanced
• Run ID: 5eb64b80-f06c-40ac-ad2a-ad651c392bc3

📥 Commits

Reviewing files that changed from the base of the PR and between fe2b59e and 8e9ff1c.

📒 Files selected for processing (5)

• .claude/skills/feature/SKILL.md
• MIGRATIONS.md
• config/defaults/myproject.config.js
• lib/helpers/mailer/README.md
• lib/helpers/mailer/index.js

Included review availability: This review used your included allowance. Your plan provides up to 1 included review per hour; 0 remain after this review.

Stale: finding fixed in cca3c6e (thread resolved, CodeRabbit confirmed in-thread); final head reviewed by Claude fallback (rate-limited).