ci(publish): create explicit GitHub release tags

[nikgraf]

Summary

Publishing now reproduces the historical GitHub release flow after npm succeeds: the workflow creates an annotated v<version> tag on the release commit, pushes that exact tag, and requires GitHub Release creation to verify the tag already exists. Changelog-derived notes and prerelease detection remain unchanged, and the contributor guide now documents the tag convention.

Validation

• Parsed the workflow as valid YAML and checked all six shell blocks with bash -n
• Confirmed stable and prerelease versions produce v1.2.3 and v1.2.3-beta.1, with --prerelease only for the latter
• Confirmed npm publish precedes tag creation and verified release creation
• pnpm lint
• pnpm test -- --run (548 passed, 4 skipped)

The next real publication remains the end-to-end verification of repository tag permissions and GitHub Release creation.