The test pyramid in practice — what to unit-test vs integration-test vs contract-test vs E2E-test vs load-test, coverage targets per layer, test data strategy, TDD where it fits, mutation testing for critical paths, and the merge-gate's coverage check. The QA Engineer agent owns this skill; every developer consumes it to know what tests their slice requires. Use whenever a slice is being planned (decide the test plan), when implementing (write the tests at the right layer), or at code review (verify tests exist at the right layer).
