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_memory_router.py

@@ -258,12 +258,13 @@ def test_update_memory_fact_route_preserves_omitted_fields() -> None:
             )
 
     assert response.status_code == 200
-    update_fact.assert_called_once_with(
-        fact_id="fact_edit",
-        content="User prefers spaces",
-        category=None,
-        confidence=None,
-    )
+    assert update_fact.call_count == 1
+    call_kwargs = update_fact.call_args.kwargs
+    assert call_kwargs.get("fact_id") == "fact_edit"
+    assert call_kwargs.get("content") == "User prefers spaces"
+    assert call_kwargs.get("category") is None
+    assert call_kwargs.get("confidence") is None
+    assert "user_id" in call_kwargs
     assert response.json()["facts"] == updated_memory["facts"]