Remove the bundled SwiftUI skill which had 13+ broken references to a
non-existent `../macos-apps/references/` directory. Add a CI script
that validates all relative .md file references in bundled skills,
preventing this class of bug from shipping again. Fix 5 additional
pre-existing broken references in other skills.
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
