feat(isolation): wire user_id through all Paths and memory callsites

Pass user_id=get_effective_user_id() at every callsite that invokes
Paths methods or memory functions, enabling per-user filesystem isolation
throughout the harness and app layers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_channel_file_attachments.py

@@ -231,7 +231,7 @@ class TestResolveAttachments:
         mock_paths = MagicMock()
         mock_paths.sandbox_outputs_dir.return_value = outputs_dir
 
-        def resolve_side_effect(tid, vpath):
+        def resolve_side_effect(tid, vpath, *, user_id=None):
             if "data.csv" in vpath:
                 return good_file
             return tmp_path / "missing.txt"