Add lint and pyright checks

[ljcc0930]

Summary

• add Ruff, Pyright, and dev requirements for repository-wide quality checks
• add a GitHub Actions workflow that runs lint and type checks on PRs and public pushes
• clean up static issues needed for the new checks to pass

Tests

• python -m ruff check .
• PYTHONPATH=/tmp/unlearn_sparse_pyright python -m pyright
• python -m compileall -q main.py src .codex/skills/unlearn-sparse-repro/scripts