Agent Skills: skill-eval

Evaluate and tune an LLM-facing instruction doc — a skill, subagent prompt, or CLAUDE.md/rule addition — by having a fresh subagent execute it blind against frozen scenarios and iterating fixes until improvements plateau. Use right after authoring or substantially revising such a doc, or when a skill misbehaved and instruction-side ambiguity is suspected rather than model failure. Not for typo-level wording changes or personal-taste polish.