Run a structured debugging session — reproduce, isolate, diagnose, and fix. Use this skill whenever the user shares an error message, stack trace, or unexpected behavior. Trigger on: "this broke", "getting an error", "not working", "something changed and now...", "works in X but not Y", "after the deploy", "this used to work", "weird behavior", "can't figure out why", or any time behavior diverges from expectations and the cause isn't obvious. Even if the user just pastes a raw error with no context — use this skill. Also trigger for performance regressions, silent failures, flaky tests, and environment-specific bugs.