0.5.2 — release script + GitHub Release procedure

[wenqingyu]

Summary

Adds a one-shot release script (`npm run release`) that handles tag-and-publish + GitHub Release creation in one go. Closes the gap that surfaced in 0.5.0 / 0.5.1: tags got pushed to origin but no GitHub Release entries were created, so the project's releases page was stuck at v0.3.9.

Changes

• `scripts/release.mjs` — automated release flow:
  • Verifies clean `main` synced with origin
  • Refuses to clobber existing tags
  • Verifies CHANGELOG has a section for the version in `package.json`
  • Runs `npm run build` (whose drift guard validates all 5 version literals match)
  • Runs `npm test`
  • Tags + pushes + `gh release create --latest` with the CHANGELOG body
• `npm run release` / `npm run release:dry-run` package scripts
• CONTRIBUTING.md — "Cutting a release" section documents the maintainer procedure end to end, calling out the five duplicated version literals (with `plugin/.claude-plugin/plugin.json` flagged as the one Claude Code's loader actually reads — that's the file 0.4.0 and 0.4.1 forgot to bump)
• Backfilled GitHub Releases for v0.4.0 → v0.5.1 with their CHANGELOG bodies (done out-of-band before this PR)

Test plan

• `npm run typecheck` — clean
• `npm test` — 117 passing
• `npm run build` — drift guard validates 5 literals at 0.5.2
• `node scripts/release.mjs --dry-run` from a non-main branch fails with the expected precondition message
• After merge: `npm run release` from `main` to dogfood the script with v0.5.2

🤖 Generated with Claude Code