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_composition_phase3.py

@@ -24,7 +24,8 @@ def test_compute_file_stats():
   py_path = os.path.join(temp_dir, "test.py")
   with open(py_path, "w") as f:
    f.write("def foo():\n    pass\n\nclass Bar:\n    pass\n")
-        
-  stats = compute_file_stats(py_path)
+  stats_result = compute_file_stats(py_path)
+  assert stats_result.ok, f"compute_file_stats failed: {stats_result.errors}"
+  stats = stats_result.data
   assert stats["lines"] == 5
   assert stats["ast_elements"] == 2 # 1 func, 1 class