feat: add vers repo command for managing repositories and repo tags

[AlephNotation]

Summary

Adds the vers repo command with full CRUD for repositories and their tags.

Commands

vers repo create <name> [-d description]
vers repo list [-q] [--format json]
vers repo get <name> [--format json]
vers repo delete <name>...
vers repo visibility <name> --public[=false]
vers repo fork <org>/<repo>:<tag> [--repo-name X] [--tag-name Y]
vers repo tag create <repo> <tag> <commit-id> [-d description]
vers repo tag list <repo> [-q] [--format json]
vers repo tag get <repo> <tag> [--format json]
vers repo tag update <repo> <tag> [--commit X] [-d description]
vers repo tag delete <repo> <tag>...

Implementation

• Uses Client.Execute directly since the Go SDK (vers-sdk-go) doesn't have repository endpoints yet. DTOs are defined locally in the CLI until SDK support lands.
• Follows existing patterns: handlers, presenters, cobra commands (mirrors vers tag structure).
• 4 new files, no modifications to existing code.

Testing

Smoke tested all endpoints against production:

• repo create / list / get / delete ✅
• repo tag create / list / get ✅
• --format json and -q output modes ✅