Add SessionService.clone_session_for_window

Opens a fresh harness session with the same project, harness, and model
as the source window, creating a new sibling hqt-<id> conversation.

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

tests/test_sessions.py

@@ -1601,3 +1601,36 @@ async def test_open_tool_window_for_window_resolves_by_name(
 async def test_open_tool_window_for_window_unknown_window_raises(service):
     with pytest.raises(ServiceError):
         await service.open_tool_window_for_window("not-an-hqt-window", "nvim")
+
+
+# ---------------------------------------------------------------------------
+# Task 5: clone_session_for_window
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_clone_session_for_window_reuses_project_harness_model(
+    service, db, monkeypatch
+):
+    await service.create_session(
+        project_id=1, harness_name="claude-code", nickname="orig", model="opus"
+    )
+    captured = {}
+
+    async def fake_create(project_id, harness_name, nickname=None, model=None):
+        captured["args"] = (project_id, harness_name, nickname, model)
+        return "SENTINEL"
+
+    monkeypatch.setattr(service, "create_session", fake_create)
+
+    result = await service.clone_session_for_window("hqt-1")
+
+    assert result == "SENTINEL"
+    # Same project + harness + model; a fresh sibling, so no nickname.
+    assert captured["args"] == (1, "claude-code", None, "opus")
+
+
+@pytest.mark.asyncio
+async def test_clone_session_for_window_unknown_window_raises(service):
+    with pytest.raises(ServiceError):
+        await service.clone_session_for_window("not-an-hqt-window")