tdd-mutation

Drives implementation with strict TDD and proves the resulting tests actually catch bugs via mutation testing. Use when implementing any feature, fixing any bug, refactoring code that has tests, or changing observable behavior. Use when the user says "implement", "add", "fix", "make it do X", "change how Y works", or asks "do my tests actually catch bugs". Do not use for pure config changes, documentation, or spike code committed for deletion.