chore: add automated changelog functionality

[schinery]

Adds automated changelog generation for releases using git-cliff and GitHub Actions.

Changes

• Workflow: New .github/workflows/changelog.yml that triggers on tag creation to auto-generate changelogs and create GitHub Releases
• Configuration: Added cliff.toml for both gems to parse commits into organized changelog sections
• Baseline: Created backfilled CHANGELOG.md documenting all past releases
• Dependabot: Updated .github/dependabot.yml to use chore: prefix for dependency updates (excluded from changelog)
• Actions: Pinned workflow actions to commit SHAs for security and reproducibility

How It Works

1. When you run rake release, it creates a git tag and pushes to GitHub
2. The workflow automatically triggers and generates a changelog
3. A GitHub Release is created with the changelog as the body
4. Dependabot updates are automatically excluded (using chore: prefix)

PR Title Format

Use Conventional Commits for PR titles when squash-merging:

• feat: - new features
• fix: - bug fixes
• docs: - documentation
• perf: - performance improvements
• refactor: - code refactoring
• test: - test changes
• ci: / chore: - skipped from changelog

Notes

• CHANGELOG.md is committed to the repo for developer reference but not included in the gem distribution
• Old releases reference GitHub releases for historical context
• The workflow will generate detailed changelogs for all future releases automatically