ci: enforce conventional commits on PRs via GitHub Actions

[alma-seele]

Summary

Adds automated commit message validation to enforce conventional commit format on all pull requests via GitHub Actions.

How It Works

The workflow:

1. Triggers on PR open, sync, or reopen events
2. Fetches the full commit history and base branch
3. Compares all commits between the base branch and PR head using git log origin/main..HEAD
4. Validates each commit message against the conventional commit pattern:

   type(optional-scope): description
   

5. Reports which commits pass/fail with clear feedback and examples

Valid types: feat, fix, docs, chore, refactor, test, ci, perf, style, revert

Changes

• .github/workflows/commitlint.yml - GitHub Actions workflow that validates commit messages
  • Fetches base branch explicitly to ensure proper comparison
  • Pure bash implementation (no external dependencies)
  • Provides helpful error messages with format examples
• CONTRIBUTING.md - Comprehensive contributing guide with:
  • Conventional commit format specification
  • Valid commit types and scopes
  • Real-world examples
  • Pull request process
• README.md - Updated to reference CONTRIBUTING.md

Example Output

✅ Valid commits pass:

✅ feat: add support for subdirectory images
✅ fix: resolve websocket connection timeout
✅ ci: add conventional commit validation

❌ Invalid commits fail with helpful feedback:

❌ Added new feature
   ^ Does not match conventional commit format

Expected format: type(optional-scope): description
Valid types: feat, fix, docs, chore, refactor, test, ci, perf, style, revert

Examples:
  feat: add support for subdirectory images
  fix: resolve websocket connection timeout

Benefits

• Enforces existing conventional commit standard automatically
• Immediate feedback to contributors during PR review
• Maintains clean, semantic commit history for changelogs
• Zero external dependencies (pure bash)
• Can be required via branch protection rules