skill-autobench — write the eval from lived usage

Convention: see conventions/brain-first.md — mining starts in the brain. Search the conversation archive before touching raw transcript files, and never declare "no history" without having queried the brain first.

Convention: see conventions/model-routing.md — mining and synthesis run on the cheap tier by default. The full multi-model judging pass is an explicit opt-in (see Contract).

The self-improving loop has three legs: an eval, a variant generator (SkillOpt), and a replay + judge harness (gbrain eval cross-modal). The generator and the judge ship with gbrain. The persistently missing leg is the eval author — someone has to WRITE the eval, and a spec-derived benchmark only tests what the skill promised, not what users actually asked for or what actually went wrong. This skill writes the eval from reality instead of imagination.