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>

lefthook.yml

@@ -0,0 +1,15 @@
+# ABOUTME: Git hooks configuration using lefthook.
+# ABOUTME: Runs ruff linting/formatting and pytest before commits.
+
+pre-commit:
+  parallel: true
+  commands:
+    ruff-check:
+      glob: "**/*.py"
+      run: ruff check src/ tests/
+    ruff-format:
+      glob: "**/*.py"
+      run: ruff format --check src/ tests/
+    pytest:
+      glob: "**/*.py"
+      run: pytest tests/ -q --tb=short