feat(gui): Merge Session Hub into Discussion Hub

- Removed Session Hub window from _gui_func
- Discussion Hub now has tab bar: Discussion | Context Composition | Snapshot | Takes
- _render_discussion_tab: history + message/response tabs
- _render_snapshot_tab: Aggregate MD + System Prompt (moved from Session Hub)
- _render_context_composition_placeholder: placeholder for Phase 3
- _render_takes_placeholder: placeholder for Phase 4

tests/test_session_hub_merge.py

@@ -0,0 +1,42 @@
+import pytest
+import inspect
+
+
+def test_session_hub_window_removed():
+    import src.gui_2 as gui_2
+
+    source = inspect.getsource(gui_2.App._gui_func)
+    assert "Session Hub" not in source, "Session Hub window should be removed"
+
+
+def test_discussion_hub_has_snapshot_tab():
+    import src.gui_2 as gui_2
+
+    source = inspect.getsource(gui_2.App._gui_func)
+    assert "Snapshot" in source, "Discussion Hub should have Snapshot tab"
+    assert "_render_snapshot_tab" in source, "Discussion Hub should call _render_snapshot_tab"
+
+
+def test_discussion_hub_has_context_composition_placeholder():
+    import src.gui_2 as gui_2
+
+    source = inspect.getsource(gui_2.App._gui_func)
+    assert "Context Composition" in source, (
+        "Discussion Hub should have Context Composition tab placeholder"
+    )
+
+
+def test_discussion_hub_has_takes_tab():
+    import src.gui_2 as gui_2
+
+    source = inspect.getsource(gui_2.App._gui_func)
+    assert "Takes" in source, "Discussion Hub should have Takes tab"
+
+
+def test_show_windows_no_session_hub():
+    import src.app_controller as app_controller
+
+    source = inspect.getsource(app_controller.AppController)
+    assert "Session Hub" not in source, (
+        "Session Hub should be removed from show_windows"
+    )