fix(pytest): restrict test discovery to tests/ directory

Root-level pytest --collect-only crashed because scripts/ contained
operational *_test.py files (local_timmy_proof_test.py,
local_decision_session_test.py) that executed code at import time.

Fix: add pyproject.toml with testpaths = ["tests"] and
python_files = ["test_*.py"] so pytest only collects from tests/.

Closes #607

pyproject.toml

@@ -0,0 +1,3 @@
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+python_files = ["test_*.py"]