Scaffolds, reviews, upgrades, or diagnoses agent skills against best-practice frontmatter, progressive disclosure, token-aware structure, and the agent-skills.git symlink + inventory wiring. Modes: `scaffold` (default — new skill), `review` (audit existing skill), `upgrade` (split a single-file skill into multi-file), `diagnose` (retrospective failure analysis that emits a confidence-gated unified diff against any skill declaring a diagnostic surface). Triggers on "create a skill", "scaffold a skill", "new SKILL.md", "review this skill", "audit my skill", "upgrade this skill", "split this skill", "diagnose this skill", "why did the skill miss this", "/create-skill".