feat(overlays): support command use rename

[samzong]

Summary

• Add an overlay use field so command overlays can rename the canonical generated command name.
• Validate final generated command paths after overlays so rename conflicts fail codegen.
• Document the overlay example and cover parsing, merge behavior, and conflict validation with tests.

Closes #62.

Verification

• make check
• git diff --cached --check

Compatibility

This is opt-in overlay behavior. Existing generated command names are unchanged unless an overlay sets commands.<old-use>.use. When set, generated Cobra command names, runtime catalog paths, and generated Skill references use the renamed command. Catalog schema is 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.