Let the release commit pass its own gates: README status version and the release-prep check

[NSchatz]

The release pipeline runs pnpm test on Changesets' version commit before it publishes. That commit moves package.json to 0.1.0 and consumes every pending changeset, and two gates asserted the pre-bump state only, so the 0.1.0 publish would have stopped in the Verify step.

Repro (before this change)

On a copy of #157's head:

pnpm install --frozen-lockfile
pnpm run version       # package.json -> 0.1.0, changesets consumed
# commit that result locally, as the Version Packages PR would
pnpm exec vitest run test/readme-standard.test.ts test/release/release-prep.test.ts
# 4 failed | 80 passed:
#   readme-standard: "names the version package.json declares" (README still says 0.0.18)
#   release-prep: "passes the release-prep check", "AC7: ... 0.0.x band", "AC3: ... resolve to exactly 0.1.0"

The change

• scripts/sync-readme-version.ts rewrites the one **Version X.Y.Z** marker in README ## Status to package.json's version, and refuses (exit 1) when the section or a single marker is missing. The version script runs it after sync-version.mjs and formats README.md, so the bump and the README line land in the same commit, as the VERSION constant already does. test/scripts/sync-readme-version.test.ts drives it on README text built in the test and checks this README carries exactly one marker.
• README ## Status now states what 0.1.0 promises, in the wording the README standard requires from 0.1.0 on ("settled and safe to depend on"), plus the stability terms: exported names and warning codes kept stable, the warning registry additions-only, and below 1.0 a breaking change bumps the minor with its migration in the changelog. Before the bump it reads "From 0.1.0 ..."; after the sync it reads "Version 0.1.0 ... From 0.1.0 ...".
• test/release/release-prep.test.ts recognizes the release commit: the manifest at the prepared version AND nothing pending (isReleaseCommit). Only there are the three prepare-state checks skipped. A premature bump (0.1.0 with the set pending) and a set removed without the bump both still fail, each pinned by a synthetic case; the section and inventory-enumeration checks run in every state.

Proof after the change

The same simulation on #157 plus this branch: pnpm run version prints sync-readme-version: README.md -> 0.1.0, and pnpm typecheck && pnpm lint && pnpm test && pnpm build && pnpm attw exits 0 on the simulated version commit (121 files, 4945 passed, 8 skipped). On this branch as pushed, pnpm typecheck, pnpm lint and the three affected test files pass (96 tests).

No change under src/. Independent of #157 in content; either can merge first.

https://claude.ai/code/session_01U24bnHKNxQWMxmfN7ny7w9