[miniflare] Add opt-in local Artifacts simulator [2/3]

[leostera]

Artifacts local-development stack — 2 of 3

Review-only fork PR. Do not merge this PR into the fork. Review in order, then merge corresponding upstream PRs in order: 1/3 — native Git client (upstream #16176) → 2/3 — opt-in simulator (this PR) → 3/3 — development tools and local default (fork #2). This PR is based on 1/3 and shows only the simulator layer. Once 1/3 merges upstream, this layer will be opened as a new upstream PR against main; then 3/3 follows. #16092 remains an unchanged full-stack reference, not another PR to merge.

What changes

• Add the Miniflare binding Worker, durable metadata, Node Git sidecar, authenticated Git smart HTTP, namespace persistence, and lifecycle management.
• Expose the 15 public binding methods via real workerd RPC, with direct Git fetch/clone and push.
• At this intermediate stage, local storage requires explicit dev.remote: false. An omitted setting retains existing remote behavior; the final development-tools layer changes the default.
• Include an internal architecture README and a Miniflare changeset. The public Artifacts docs PR remains draft until the final behavior and release are approved.

The simulator builds on the original prototype by @GregBrimble. Focused local tests establish tested behavior, not parity with the deployed service. The guarded live comparator has not been run: it needs authorization for an exclusively reserved disposable namespace. Explicit HTTPS imports can contact the supplied host, including private-network destinations, and do not follow HTTP redirects.

Validation

• pnpm -r --filter miniflare... build
• pnpm --filter miniflare exec vitest run test/plugins/artifacts test/config/v4-convert.spec.ts — 82 focused tests passed (including inherited Git-client tests)
• Miniflare typecheck, type-aware lint, format, and changeset validation passed

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• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s): [miniflare] Add local Artifacts binding simulator cloudflare/cloudflare-docs#34106
  • Documentation not necessary because:

[leostera]

Closing this fork-only review PR as requested. The replacement will target cloudflare/workers-sdk directly; the branch and commits remain available.