Scaffolds, diagnoses, and prunes agent skills. Modes: `scaffold` (default — runs the job once without a skill on a real input, records every correction, then writes a skill made of only those corrections), `diagnose` (retrospective failure analysis that emits a confidence-gated unified diff against any skill declaring a diagnostic surface), `prune` (deletes unused skills and trims or deletes those whose with/without-skill eval shows no lift over a same-length dummy). Triggers on "create a skill", "scaffold a skill", "new SKILL.md", "diagnose this skill", "why did the skill miss this", "prune my skills", "clean up my skills", "/create-skill".