Add monopam verse fork command for forking packages from verse members
New command: monopam verse fork <package> --from <handle> --url <fork-url>
This allows users to fork a package from a verse member's repository:
- Looks up the package in verse/<handle>-opam/
- Finds all packages sharing the same git repository
- Creates entries in user's opam-repo with fork URL as dev-repo
- Supports --dry-run to preview what would be forked
- Detects conflicts if packages already exist in user's opam-repo
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>