Diagnose

A disciplined loop for hard code bugs and performance regressions: something throws, produces NaNs, hangs, or has measurably regressed. Use when asked to "debug this" or "diagnose this", when a test is failing or a process is crashing, when a metric or latency has regressed unexpectedly, or when a third-party call fails on real data (the general home for the minimal-repro move that experiment-workflow cites in its "When a third-party call fails" item). Boundary: this skill owns a code FAILURE; a surprising result (a number too good or too bad) is the "is it our bug?" gate in experiment-workflow, load that skill instead.