newskill — add a skill without drift

/newskill scaffold <name> "<description>" <category> — create a new skill, registered /newskill fix [<name>] — complete a skill that exists in some surfaces but not others

A skillhub skill lives in four surfaces, and it is only really "added" when all four agree:

1. Folder — .claude/skills/<name>/SKILL.md with valid frontmatter.
2. README table row — a row in the right category table (a prose mention does not count; the validator checks for a table row specifically).
3. Marketplace — <name> listed in one plugin's skills[] in .claude-plugin/marketplace.json.
4. Trigger evals — cases in evals/triggers.json (model-invoked skills only; user-invoked skills are exempt).