chore(#556): retire orphaned CJS↔SDK hand-sync tooling (#559)

The @opengsd/gsd-sdk package boundary was retired (ADR-0174, #191/#192) and
the sdk/ tree is no longer tracked. ADR-0174's Supersedes table explicitly
records that the generator-based Shared-Module hand-sync lint is deleted as
part of that collapse. This removes the now-orphaned machinery it left behind:

- scripts/lint-shared-module-handsync.cjs — paired bin/lib/*.cjs files with
  sdk/src/**/*.ts sources that no longer exist; wired into no CI workflow or
  npm script (dead).
- scripts/shared-module-handsync-allowlist.json — the lint's allowlist; every
  entry pointed at a non-existent sdk/src source / generated artifact /
  freshness check.
- tests/lint-shared-module-handsync.test.cjs — tested the deleted lint.

Docs corrected to match:
- CONTRIBUTING.md — removed the "CJS↔SDK seam" instruction (it linked the
  already-deleted docs/agents/cjs-sdk-seam.md and told contributors to
  maintain the allowlist under a retired generator pattern).
- docs/prd/3524-cjs-sdk-hard-seam.md + docs/prd/README.md — marked the PRD
  Superseded by ADR-0174, matching the already-superseded ADR-3524.

Added tests/no-cjs-sdk-handsync-tooling.test.cjs as a regression guard so the
retired tooling stays removed and is not silently re-wired into package.json.

ADR-3524 is left in place (already Superseded by ADR-0174); runtime modules and
regression tests that cite it in comments keep resolving. No user-facing change.

Closes #556

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/prd/3524-cjs-sdk-hard-seam.md

@@ -1,6 +1,6 @@
 # PRD: CJS↔SDK hard seam — Shared-Module migration
 
-- **Status:** Reference
+- **Status:** Superseded by [ADR-0174](../adr/0174-retire-gsd-sdk-package-boundary.md) (2026-05-23) — historical migration plan; the CJS↔SDK seam and its hand-sync tooling were retired with the `@opengsd/gsd-sdk` package boundary
 - **Date:** 2026-05-14
 - **Tracking issue:** [#3524](https://github.com/open-gsd/gsd-core/issues/3524)
 - **Related ADR:** [`docs/adr/3524-cjs-sdk-hard-seam.md`](../adr/3524-cjs-sdk-hard-seam.md)

docs/prd/README.md

@@ -24,4 +24,4 @@ The GitHub-assigned issue number is the prefix. Do not compute a sequential numb
 
 | PRD | Title | Status |
 |-----|-------|--------|
-| [3524-cjs-sdk-hard-seam.md](3524-cjs-sdk-hard-seam.md) | CJS↔SDK hard seam — phased migration (#3524) | Reference |
+| [3524-cjs-sdk-hard-seam.md](3524-cjs-sdk-hard-seam.md) | CJS↔SDK hard seam — phased migration (#3524) | Superseded by ADR-0174 |