描述
Systematic 4-phase debugging: (1) understand the failure by reading the complete error and reproducing it, (2) find working examples to compare against, (3) test one hypothesis at a time with a single change, (4) fix the root cause and verify with tests. Includes the Three Strikes Rule (escalate after 3 failed fix attempts) and the Iron Law (no fixes without investigation). Activate when tests fail unexpectedly, errors occur, behavior is wrong, something that worked before is now broken, or a user reports a bug. Triggers on: "debug", "why is this failing", "test failure", "unexpected error", "bug", "broken", "investigate this error", "fix this bug", "tests are failing", "weird behavior", "something broke", "track down this issue". Also activates when multiple bugs need systematic investigation (one at a time, not batch-fixed). NOT for: writing new tests from scratch (use tdd), or general code review (use code-review).