Use when writing, refactoring, or extending code in any repo and the user wants strict working rules applied throughout - no comments, one responsibility per unit, constants and types in their own files, reuse existing helpers before writing new ones, prefer the repo's utility and date libraries, no dead code or magic values, strict types, handled edge cases and errors, tests and lint always green, no mocking the module under test, ask instead of assume, verify library APIs against current docs, and surface decisions to the user.
