chore: prevent package-lock.json version drift permanently

Makes the drift that PR #70 caught impossible to repeat:

- `version-packages` script now runs `changeset version && npm install
  --package-lock-only && check-lockfile-sync`, so the lockfile is always
  regenerated and verified as part of consuming a changeset
- New `scripts/check-lockfile-sync.mjs` compares package.json#.version against
  package-lock.json's root and packages[""] version fields (npm ci does not
  enforce these, which is why the prior drift slipped through CI)
- CI now runs `npm run check:lockfile` on every push/PR — any future drift
  fails the build before merge
- COM workflow in CLAUDE.md collapsed back to a single release-bump step now
  that lockfile sync is automatic

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>