chore: add pre-commit hook with ruff and pytest

- Add git pre-commit hook running ruff check, ruff format, pytest
- Fix import ordering in conftest.py
- Add placeholder test for test infrastructure
- Generate flake.lock

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

tests/test_placeholder.py

@@ -0,0 +1,7 @@
+# ABOUTME: Placeholder test to ensure test infrastructure works.
+# ABOUTME: Remove once real tests are added.
+
+
+def test_placeholder():
+    """Placeholder test - remove when real tests exist."""
+    assert True