chore: adopt release-please for automated versioning

[lukas-grigis]

Summary

Replace the manual version bump + tag workflow with release-please for fully automated versioning.

Current flow (manual)

1. Bump version in package.json
2. Update CHANGELOG.md
3. Commit, tag, push
4. Pipeline publishes

Target flow (release-please)

1. Merge PRs with conventional commits
2. release-please automatically maintains a "Release PR" that bumps version + CHANGELOG
3. Merge the Release PR → tag is created → pipeline publishes to npm + creates GitHub Release

Why

• Zero manual version management
• CHANGELOG is always up to date
• Fits the existing conventional commit convention
• Used by Firebase, Angular, Google Cloud, and many npm packages

Implementation

• Add .github/workflows/release-please.yml
• Configure for Node.js / npm package type
• Update CONTRIBUTING.md releasing section
• Existing release.yml workflow continues to handle npm publish on tag push (release-please creates the tags)

Priority

Low — current manual flow works fine for now. Adopt when release frequency increases.