Enforces strict test-driven development -- write a failing test first, implement minimal code to pass, then refactor. Use when implementing a feature, fixing a bug, adding behavior, refactoring, or any time production code will be written or changed. Also use when the user says "write tests", "add tests", "TDD", or "test first". DO NOT TRIGGER for throwaway prototypes, generated code, or config-only changes unless the user explicitly requests TDD.