chore: migrate to lefthook for git hooks

Replace custom bash pre-commit hook with lefthook for:
- Parallel execution of ruff + pytest
- Quieter output (pytest -q instead of -v)
- Version-controlled config (lefthook.yml)
- Smart skipping when no .py files staged

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -46,3 +46,4 @@ build/
 .env
 *.local
 .sesskey
+lefthook-local.yml