test(sandbox): update v3 paths-aware tests for FR1+FR3 invariants

- test_paths.py: explicit initialize_paths(<empty_config>) instead of
  SLOP_CONFIG env var (v3 design); add restore_paths fixture so other
  tests keep their conftest workspace init.
- test_summary_cache.py: use tmp_path (under ./tests/) instead of
  hardcoded Path('.test_cache') that FR1 blocks.
- test_orchestrator_pm_history.py: use tempfile.mkdtemp() instead of
  writing to project-root 'test_conductor/' that FR1 blocks.
- test_gui_paths.py::test_save_paths: mock src.paths.initialize_paths
  instead of src.paths.reset_paths (v3 entry point).

All 12 tests pass in the Tier 2 clone after these fixes.

tests/test_gui_paths.py

@@ -26,7 +26,7 @@ def test_save_paths():
  
  with patch('shutil.copy') as mock_copy, \
       patch('src.paths.get_config_path') as mock_get_cfg, \
-      patch('src.paths.reset_paths') as mock_reset, \
+      patch('src.paths.initialize_paths') as mock_init_paths, \
       patch.object(MockApp, 'init_state') as mock_init:
   
   mock_get_cfg.return_value = MagicMock()
@@ -40,5 +40,5 @@ def test_save_paths():
   mock_app.save_config.assert_called_once()
   mock_copy.assert_called_once()
   assert 'applied' in mock_app.ai_status
-  mock_reset.assert_called_once()
+  mock_init_paths.assert_called_once()
   mock_init.assert_called_once()