feat(gui): Implement Context Composition panel (Phase 3)

- Replaced placeholder with actual _render_context_composition_panel
- Shows current files with Auto-Aggregate and Force Full flags
- Shows current screenshots
- Preset dropdown to load existing presets
- Save as Preset / Delete Preset buttons
- Uses existing save_context_preset/load_context_preset methods

tests/test_context_composition_panel.py

@@ -0,0 +1,42 @@
+import pytest
+import inspect
+
+
+def test_context_composition_panel_replaces_placeholder():
+    import src.gui_2 as gui_2
+
+    source = inspect.getsource(gui_2.App._gui_func)
+    assert "_render_context_composition_placeholder" not in source, (
+        "Placeholder should be replaced"
+    )
+    assert "_render_context_composition_panel" in source, (
+        "Should have _render_context_composition_panel"
+    )
+
+
+def test_context_composition_has_save_load_buttons():
+    import src.gui_2 as gui_2
+
+    source = inspect.getsource(gui_2.App._render_context_composition_panel)
+    assert "Save as Preset" in source or "save" in source.lower(), (
+        "Should have Save functionality"
+    )
+    assert "Load Preset" in source or "load" in source.lower(), (
+        "Should have Load functionality"
+    )
+
+
+def test_context_composition_shows_files():
+    import src.gui_2 as gui_2
+
+    source = inspect.getsource(gui_2.App._render_context_composition_panel)
+    assert "files" in source.lower() or "Files" in source, "Should show files"
+
+
+def test_context_composition_has_preset_list():
+    import src.gui_2 as gui_2
+
+    source = inspect.getsource(gui_2.App._render_context_composition_panel)
+    assert "context_presets" in source or "preset" in source.lower(), (
+        "Should reference presets"
+    )