feat: add Release Please workflow

[ineedjet]

🤖 [rubykatzen/baseline]:

Summary

• bootstrap Release Please from v0.9.0 with the Ruby strategy
• publish the gem and advance floating major/minor tags after release creation
• update self-references through generic version annotations
• remove the legacy releaser workflows and their unused revision-bump action
• document the GitHub-native release flow

Tracks #126.

Setup before merge

• create a fine-grained PAT limited to rubykatzen/baseline
• grant read/write access to contents, issues, and pull requests
• save it as repository secret RELEASE_TOKEN

Verification

• python3 -m unittest discover -s test -v
• pre-commit run --all-files
• release-please release-pr --dry-run against this branch

🚀 📦 ✅