init: common-skills v1

skills/testing/SKILL.md

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+---
+name: testing-skill
+description: Testing best practices for Python and general projects. Use when writing unit tests, debugging test failures, or improving test coverage.
+---
+
+# Testing Skill
+
+## Test Structure (AAA Pattern)
+
+```python
+def test_decision_layer_returns_action():
+    # Arrange
+    state = {"health": 80, "enemy_visible": True}
+
+    # Act
+    action = decision_layer.decide(state)
+
+    # Assert
+    assert action == "attack"
+```
+
+## Pytest Tips
+
+### Parametrize to Avoid Duplication
+```python
+import pytest
+
+@pytest.mark.parametrize("health,expected", [
+    (100, "idle"),
+    (30,  "flee"),
+    (0,   "dead"),
+])
+def test_state_by_health(health, expected):
+    assert get_state(health) == expected
+```
+
+### Use Fixtures for Shared Setup
+```python
+@pytest.fixture
+def mock_vision():
+    return {"objects": ["enemy", "wall"], "confidence": 0.95}
+
+def test_understanding_layer(mock_vision):
+    result = understanding_layer.parse(mock_vision)
+    assert "enemy" in result["threats"]
+```
+
+## What to Test
+- ✅ Happy path (normal input)
+- ✅ Edge cases (empty, None, boundary values)
+- ✅ Error paths (invalid input raises expected exception)
+- ❌ Don't test implementation details — test behavior