feat(overlays): expose structured command examples

[samzong]

Summary

Adds structured overlay examples metadata for generated commands and carries it through generated CommandSpec, the runtime catalog, and generated Skill references.

Keeps legacy example projected into examples for catalog consumers, documents the overlay shape, and bumps the catalog schema to 7.

Closes #72

Verification

• make check
• git diff HEAD~1..HEAD --check
• pre-ship committed diff scan: 0 MUST-FIX

Compatibility

Catalog schema changes from 6 to 7. commands --json and commands show --json now include examples when overlays define structured example metadata or legacy example is present.

Runtime execution, auth, body handling, output formatting, and generated output commit policy are unchanged.

Checklist

• Tests or focused verification cover the changed surface.
• User-facing behavior changes are documented.
• Generated output under internal/generated/, .cache/, and ad-hoc skills/<cli-name>/ directories is not committed.
• Commits are signed off when this is ready to merge.

[samzong]

@codex review