Agent Skills: create-skill

Single entry for authoring, auditing, and optimizing Codex skills under .codex/skills/NAME. Three lanes: Create scaffolds a folder and SKILL.md template; Audit runs deterministic checks over frontmatter, naming, body size, description quality, and reference integrity; Optimize fixes audit findings such as description rewrites, body to references split, frontmatter repair, and name normalization. Use when the operator says create a skill, scaffold a skill, add SKILL.md, audit a skill, check skills, validate skill conformance, optimize a skill, fix a skill description, shrink a skill, or why is this skill not activating.