docs(tmux-detector): add comprehensive detector poller design
Specifies the complete architecture for the harness-agnostic tmux
detector poller, answering all key questions:
- Opt-in mechanism: @tb- pane title prefix pattern
- Quiet detection: capture-pane hash comparison
- Prompt detection: configurable regex patterns
- Quiet threshold: 30 seconds (configurable)
- Unstuck detection: any output change
- Session ID generation: synthetic tmux-{paneId}-{timestamp}
Includes state machine, poll loop pseudocode, error handling,
daemon integration, testing strategy, and future enhancements.
Addresses bead tb-1ke: tmux detector poller architecture design.
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