feat(dashboard): tmux split-pane helpers + Enter dispatch

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`).

tests/unit/test_dashboard_active_agents.py

@@ -379,6 +379,65 @@ class TestBottleForSlug(unittest.TestCase):
         self.assertEqual("", hint)
 
 
+class TestTmuxPaneArgvBuilders(unittest.TestCase):
+    """Pure argv builders for the tmux split-pane integration
+    (PRD 0021 chunk 2). The subprocess invocation itself is
+    environment-dependent; here we lock the wrapping shape so
+    a regression surfaces in CI without needing a real tmux."""
+
+    DOCKER_ARGV = [
+        "docker", "exec", "-it",
+        "claude-bottle-dev-abc",
+        "claude", "--dangerously-skip-permissions", "--continue",
+    ]
+
+    def test_split_pane_argv_horizontal_with_pane_id_capture(self):
+        argv = dashboard._build_split_pane_argv(self.DOCKER_ARGV)
+        self.assertEqual(
+            ["tmux", "split-window", "-h",
+             "-P", "-F", "#{pane_id}",
+             *self.DOCKER_ARGV],
+            argv,
+        )
+
+    def test_respawn_pane_argv_kills_existing_process(self):
+        argv = dashboard._build_respawn_pane_argv("%12", self.DOCKER_ARGV)
+        self.assertEqual(
+            ["tmux", "respawn-pane", "-k", "-t", "%12", *self.DOCKER_ARGV],
+            argv,
+        )
+
+    def test_respawn_pane_argv_threads_pane_id_unmodified(self):
+        # Pane ids contain `%`; make sure we pass them straight
+        # through to `-t` without quoting or substitution surprises.
+        argv = dashboard._build_respawn_pane_argv("%abc.123", ["sh"])
+        self.assertIn("%abc.123", argv)
+
+
+class TestClaudeRuntimeArgs(unittest.TestCase):
+    """The argv passed to `bottle.claude_docker_argv` on each
+    attach. Locked here so the tmux + foreground paths build
+    identical claude invocations."""
+
+    def test_default_skip_permissions_only(self):
+        self.assertEqual(
+            ["--dangerously-skip-permissions"],
+            dashboard._claude_runtime_args(resume=False),
+        )
+
+    def test_resume_appends_continue(self):
+        self.assertEqual(
+            ["--dangerously-skip-permissions", "--continue"],
+            dashboard._claude_runtime_args(resume=True),
+        )
+
+    def test_remote_control(self):
+        args = dashboard._claude_runtime_args(
+            resume=False, remote_control=True,
+        )
+        self.assertIn("--remote-control", args)
+
+
 class TestStopBottleFlow(unittest.TestCase):
     """Explicit per-bottle stop (PRD 0020 chunk 4). The non-owned
     path is the one safe to test without curses + docker — the