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/backend/docker/bottle.py

@@ -28,7 +28,15 @@ class DockerBottle(Bottle):
         self._prompt_path = prompt_path_in_container
         self._closed = False
 
-    def exec_claude(self, argv: list[str], *, tty: bool = True) -> int:
+    def claude_docker_argv(
+        self, argv: list[str], *, tty: bool = True,
+    ) -> list[str]:
+        """Return the full `docker exec` argv for running claude in
+        this bottle. Public so callers that want to spawn claude
+        somewhere other than the dashboard's foreground (e.g.,
+        `tmux new-window` from the dashboard when `$TMUX` is set)
+        can build on the same command without duplicating the
+        `--append-system-prompt-file` plumbing."""
         full_argv = list(argv)
         if self._prompt_path:
             full_argv.extend(["--append-system-prompt-file", self._prompt_path])
@@ -36,7 +44,12 @@ class DockerBottle(Bottle):
         if tty:
             cmd.append("-it")
         cmd.extend([self.name, "claude", *full_argv])
-        return subprocess.run(cmd, check=False).returncode
+        return cmd
+
+    def exec_claude(self, argv: list[str], *, tty: bool = True) -> int:
+        return subprocess.run(
+            self.claude_docker_argv(argv, tty=tty), check=False,
+        ).returncode
 
     def exec(self, script: str) -> ExecResult:
         # Pipe via stdin to `sh -s` so the caller never has to worry