Add remaining crew subcommands for user-managed workspaces:
- gt crew list: List crew workspaces with session/git status
- gt crew at/attach: Start or attach to tmux session
- gt crew remove: Remove workspace with safety checks
- gt crew refresh: Context cycling with mail-to-self handoff
- gt crew status: Show detailed workspace status with mail info
All commands support --rig flag for explicit rig selection and
--json output where appropriate.
Closes: gt-cik.9
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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