Add TmuxManager.open_aux_window delegate

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_tmux.py

@@ -1083,3 +1083,14 @@ async def test_new_aux_window_kills_window_when_set_option_fails(runner):
     assert wid is None
     # the half-built window must be cleaned up by id.
     assert runner._exec.call_args_list[-1].args == ("kill-window", "-t", "@7")
+
+
+@pytest.mark.asyncio
+async def test_manager_open_aux_window_delegates(runner):
+    from unittest.mock import AsyncMock
+
+    runner.new_aux_window = AsyncMock(return_value="@4")
+    mgr = TmuxManager(runner)
+    wid = await mgr.open_aux_window("nvim", "/p", ["nvim"], "nvim · p")
+    assert wid == "@4"
+    runner.new_aux_window.assert_awaited_once_with("nvim", "/p", ["nvim"], "nvim · p")