Add missing gt commands to match PGT functionality:
- gt session restart: Restart polecat session (stop + start)
- gt session status: Show detailed session status with uptime
- gt rig shutdown: Gracefully stop all agents in a rig
- gt mail reply: Convenience command for replying to messages
- gt witness attach: Attach to witness tmux session
Closes: gt-hw6, gt-99m, gt-6db, gt-e76, gt-hzr, gt-sqi
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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