fix(repo): address clone review nits
Commit

- error message matches accepted ref form (leading slash optional)
- self-contained github provider const (drop checkpoint-named borrow)
- not-found hint uses canonical /gh/<owner>/<repo>
- validate server-provided cluster host before building the clone URL
- update repo group doc comment now that clone lives there
Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com Entire-Checkpoint: 5cb16b9c7fcc
Checkpoints
Checkpoint 1