docs(tb-5qz): record tmux detector viability verdict in decisions
- Add comprehensive Tmux Detector Viability section (2026-07-02)
- Verdict: VIABLE — works as designed as universal fallback
- Document false positive rate: low (30s threshold, prompt patterns, hash comparison)
- Document false negative rate: user-dependent (opt-in compliance)
- Document performance: minimal (<50ms/cycle for 10 panes)
- Document tuning applied: quiet threshold, poll interval, opt-in prefix, prompt patterns
- Include comparison table to hook-based detection
- Resolve Open question 1: tmux-level detector is viable
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