PRD 0021 chunk 2. New tmux integration: when `\$TMUX` is set
and the operator presses Enter on a focused agent row, the
dashboard spawns / respawns the right pane with that bottle's
claude session instead of taking over the terminal via
curses.endwin.
Mechanics:
- `_in_tmux()` — true when `\$TMUX` is set.
- `_tmux_split_pane_create` — first attach: `tmux split-window
-h -P -F '#{pane_id}'` opens a right pane and prints its id
for tracking.
- `_tmux_respawn_pane` — subsequent attaches: `tmux
respawn-pane -k -t <id>` swaps the content without
re-splitting.
- `_tmux_pane_exists` — `tmux list-panes` check before
respawn so a manually-closed pane gracefully falls back to
a fresh split.
- `_attach_in_tmux` — owns the create-or-respawn state
machine, mutates `tmux_state` ({pane_id, slug}) so the
main loop tracks the right-pane occupant.
- `_attach_via_handoff` — the previous curses-endwin path,
extracted as the fallback when tmux is missing or fails.
- `_attach_to_bottle` dispatches: in tmux + state available →
`_attach_in_tmux`; otherwise → handoff.
Main loop gets `tmux_state: dict = {"pane_id": None, "slug":
None}`. Chunks 3 + 4 wire it through the new-agent flow and
the stop hook.
`FileNotFoundError`-safe `subprocess.run` calls around every
tmux invocation — a missing tmux binary cleanly falls back to
the handoff for that keypress. 478 unit tests pass (10 new
for the pure argv builders + `_claude_runtime_args`).
didericis
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