Align tmux observer output with screen.jsonl format
- Output per-session files: tmux_{session}_screen.jsonl
- Entry format now matches screen.jsonl (frame_id, timestamp, analysis, etc.)
- Add tmux category with formatter for markdown rendering (strips ANSI)
- Existing *_screen.jsonl pattern handles indexing automatically
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>