Use for 'why does X work this way', 'why we picked Y', design rationale, regressions, postmortems, or data-backed thresholds. Discovers available MCPs and queries each evidence category (source control, issue tracker, long-form docs, real-time chat, infrastructure observability, error tracking, product analytics warehouse) in parallel, then returns a cited read on decisions and tradeoffs. DO NOT use for explaining current runtime behavior (use how instead), or for proving a change is safe to ship (use blast-radius instead). Triggers: why does X work this way, why did we pick Y over Z, what motivated this, why does this still exist, history of X.