chore: Add agentic coding tooling

.crosslink/rules/python.md

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+### Python Best Practices
+
+#### Code Style
+- Follow PEP 8 style guide
+- Use type hints for function signatures
+- Use `black` for formatting, `ruff` or `flake8` for linting
+- Prefer `pathlib.Path` over `os.path` for path operations
+- Use context managers (`with`) for file operations
+
+#### Error Handling
+```python
+# GOOD: Specific exceptions with context
+def read_config(path: Path) -> dict:
+    try:
+        with open(path, 'r', encoding='utf-8') as f:
+            return json.load(f)
+    except FileNotFoundError:
+        raise ConfigError(f"Config file not found: {path}")
+    except json.JSONDecodeError as e:
+        raise ConfigError(f"Invalid JSON in {path}: {e}")
+
+# BAD: Bare except or swallowing errors
+def read_config(path):
+    try:
+        return json.load(open(path))
+    except:  # Don't do this
+        return {}
+```
+
+#### Security
+- Never use `eval()` or `exec()` on user input
+- Use `subprocess.run()` with explicit args, never `shell=True` with user input
+- Use parameterized queries for SQL (never f-strings)
+- Validate and sanitize all external input
+
+#### Dependencies
+- Pin dependency versions in `requirements.txt`
+- Use virtual environments (`venv` or `poetry`)
+- Run `pip-audit` to check for vulnerabilities
+
+#### Testing
+- Use `pytest` for testing
+- Aim for high coverage with `pytest-cov`
+- Mock external dependencies with `unittest.mock`