fix(src): update load_context_preset to handle Result from load_all

After migrating ContextPresetManager.load_all to return Result[Dict],
the caller in app_controller.load_context_preset needs to extract
.data from the Result before checking 'name not in presets'.

Updates:
- src/app_controller.py:load_context_preset - check result.ok and
  extract result.data before iterating; raise RuntimeError if
  result.ok is False (consistent with the convention).
- tests/test_context_presets_manager.py:test_manager_load_all -
  extract result.data before assertions.

Tests verified:
- tests/test_context_presets_manager.py (4 tests) PASS
- tests/test_project_switch_persona_preset.py::
  test_load_context_preset_missing_raises_keyerror PASS (KeyError
  raised correctly when preset not found)
- tests/test_phase6_engine.py (3 tests) PASS

scripts/tier2/artifacts/result_migration_small_files_20260617/apply_edit.py

@@ -0,0 +1,9 @@
+with open('src/app_controller.py', 'r', encoding='utf-8') as f:
+    content = f.read()
+old = " def load_context_preset(self, name: str) -> models.ContextPreset:\n  presets = self.context_preset_manager.load_all(self.project)\n  if name not in presets:\n   raise KeyError(f\"Context preset '{name}' not found.\")\n  preset = presets[name]"
+new = " def load_context_preset(self, name: str) -> models.ContextPreset:\n  presets_result = self.context_preset_manager.load_all(self.project)\n  if not presets_result.ok:\n   raise RuntimeError(f\"Failed to load context presets: {presets_result.errors}\")\n  presets = presets_result.data\n  if name not in presets:\n   raise KeyError(f\"Context preset '{name}' not found.\")\n  preset = presets[name]"
+assert old in content, 'old not found'
+content = content.replace(old, new)
+with open('src/app_controller.py', 'w', encoding='utf-8') as f:
+    f.write(content)
+print('Done')