Implements the core Witness functionality:
- gt witness start: Creates tmux session with Claude, theming, auto-priming
- gt witness stop: Kills tmux session and updates state
- gt witness status: Shows session state reconciled with tmux
- Shutdown handler: Verifies git clean state before cleanup, sends nudges
- Auto-spawn: Spawns polecats for ready work up to configurable capacity
- Health checks: Monitors polecat activity, nudges stuck workers, escalates
Also updates handoff to include polecat name in lifecycle requests.
Closes: gt-53w6, gt-mxyj, gt-5wtw, gt-cpm2, gt-es1i
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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