feat(dashboard): spawn claude in new tmux window when \$TMUX is set

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`).

claude_bottle/cli/dashboard.py

@@ -690,18 +690,60 @@ def _stop_bottle_flow(
     return f"[{slug}] stopped"
 
 
+def _in_tmux() -> bool:
+    """True when the dashboard is running inside a tmux session.
+    Tmux sets `$TMUX` to the path of its server socket; if it's
+    set, we can shell out to `tmux new-window` to spawn claude
+    sessions as siblings of the dashboard pane."""
+    return bool(os.environ.get("TMUX"))
+
+
+def _build_tmux_attach_argv(docker_argv: list[str], slug: str) -> list[str]:
+    """Wrap a docker-exec argv with `tmux new-window -n <slug>` so
+    the spawned claude session lives in its own tmux window
+    named after the bottle. Pure function — easy to unit-test
+    without shelling out to tmux."""
+    return ["tmux", "new-window", "-n", slug, *docker_argv]
+
+
+def _attach_via_tmux(
+    bottle,
+    slug: str,
+    *,
+    resume: bool,
+    remote_control: bool = False,
+) -> str:
+    """Spawn claude inside `bottle` in a new tmux window. Returns
+    immediately — the dashboard stays running in its current
+    tmux pane and the operator switches to the new window via
+    tmux's normal nav (default `C-b n`)."""
+    claude_args = ["--dangerously-skip-permissions"]
+    if remote_control:
+        claude_args.append("--remote-control")
+    if resume:
+        claude_args.append("--continue")
+    docker_argv = bottle.claude_docker_argv(claude_args)
+    argv = _build_tmux_attach_argv(docker_argv, slug)
+    result = subprocess.run(
+        argv, capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        err = (result.stderr or "").strip() or "unknown tmux error"
+        return f"tmux new-window failed: {err}"
+    return f"[{slug}] opened in new tmux window"
+
+
 def _attach_to_bottle(
     stdscr: "curses._CursesWindow",
     bottle,
     slug: str,
 ) -> str:
-    """Handoff: curses.endwin → attach claude → curses refresh.
-    Re-entry into a running bottle from the dashboard always
-    passes `--resume` so claude picks up its prior conversation
-    rather than starting a fresh transcript — the first attach
-    happens via `_new_agent_flow` which sets up the transcript
-    in the first place. Returns the post-attach status-line
-    message."""
+    """Re-attach to a running bottle. In tmux mode the claude
+    session opens in a new tmux window (dashboard keeps running
+    in this pane); without tmux it's a curses-endwin handoff that
+    blocks until the operator exits claude."""
+    if _in_tmux():
+        return _attach_via_tmux(bottle, slug, resume=True)
     curses.endwin()
     try:
         exit_code = attach_claude(bottle, remote_control=False, resume=True)
@@ -761,8 +803,7 @@ def _new_agent_flow(
         backend = get_bottle_backend()
         # Launch step writes to stderr (image build, network create,
         # compose up). Get out of curses' way for the duration so
-        # the lines render cleanly. The handoff stays endwin'd until
-        # claude exits, then we refresh.
+        # the lines render cleanly.
         curses.endwin()
         try:
             cm = backend.launch(plan)
@@ -773,6 +814,13 @@ def _new_agent_flow(
             raise
         bottles[plan.slug] = (cm, bottle, identity)
 
+        if _in_tmux():
+            # Bottle is up; spawn claude in a new tmux window and
+            # let the dashboard come back to the foreground in this
+            # pane. First-attach so no `--continue`.
+            stdscr.refresh()
+            return _attach_via_tmux(bottle, plan.slug, resume=False)
+
         try:
             exit_code = attach_claude(bottle, remote_control=False)
             capture_session_state(identity, exit_code)