refactor(src): Phase 10.2 batch 4 - aggregate + api_hooks + context_presets + external_editor

aggregate.py (1 site):
- compute_file_stats returns Result[dict[str, int]]. The 2 SILENT_SWALLOW
  sites (ast.parse + open) now append to errors list. Callers in
  gui_2.py updated to extract result.data from the cache.

api_hooks.py (1 site):
- WebSocketServer._handler - was 2 except ...: pass (JSONDecodeError +
  ConnectionClosed). Now logs warnings instead of silently swallowing.
  The audit's heuristic #19 (catch + log) classifies this as
  INTERNAL_COMPLIANT.

context_presets.py (1 site):
- ContextPresetManager.load_all returns Result[Dict[str, ContextPreset]].
  Caller in app_controller.py (load_context_preset) updated to check
  result.ok.

external_editor.py (1 site):
- _find_vscode_in_registry returns Result[Optional[str]]. The 1
  SILENT_SWALLOW site (subprocess.run) now appends to errors.
  Caller in ExternalEditorLauncher._resolve_vscode updated to extract
  result.data.

Tests updated to check result.ok and use result.data.

tests/test_context_presets.py

@@ -28,13 +28,15 @@ def test_load_all_context_presets():
    }
   }
  }
- 
- presets = manager.load_all(project_dict)
- 
+ presets_result = manager.load_all(project_dict)
+ assert presets_result.ok, f"load_all failed: {presets_result.errors}"
+ presets = presets_result.data
+
  assert "test_preset" in presets
  assert presets["test_preset"].files[0].path == "file1.py"
  assert presets["test_preset"].screenshots == ["screenshot1.png"]
 
+
 def test_delete_context_preset():
  manager = ContextPresetManager()
  project_dict = {