- Add find_relative_pin() to detect relative pins (not just absolute)
- ExploitingPinDetector Type 4: attack on relatively-pinned piece
  (e.g. Bb5 piles on Nd7 pinned by Qd3 to Qd8 on d-file)
- PinDetector: skip equal-value pin when target is already relatively pinned
- Add GitHub Actions: test.yml (Python 3.10-3.13) + publish.yml (PyPI trusted publisher)
- Bump version to 0.1.1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>