Option 3 from `docs/research/claude-code-pane-in-dashboard.md`,
opt-in by environment. When the dashboard runs inside tmux
(\$TMUX is set), both the new-agent (`n`) attach AND the
re-attach (Enter) paths spawn claude with
`tmux new-window -n <slug> docker exec -it … claude …` instead
of taking over the terminal via `curses.endwin`. The dashboard
keeps rendering in its current tmux pane; the operator switches
to the new window via tmux's normal nav.
Outside tmux the existing handoff path is unchanged — the
dispatch is a single `_in_tmux()` check per attach.
Mechanics:
- `DockerBottle.claude_docker_argv` extracted from `exec_claude`,
so both subprocess.run AND `tmux new-window` can build on the
same docker-exec argv (preserving `--append-system-prompt-file`).
- `_attach_via_tmux` in dashboard.py wraps the docker argv with
`tmux new-window -n <slug> …` and returns immediately. Status
line: `[slug] opened in new tmux window`.
- `_build_tmux_attach_argv` split out as a pure helper so the
wrapping shape is unit-tested without shelling out.
467 unit tests pass (2 new for `_build_tmux_attach_argv`).
didericis
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