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/app/gateway/path_utils.py

@@ -5,6 +5,7 @@ from pathlib import Path
 from fastapi import HTTPException
 
 from deerflow.config.paths import get_paths
+from deerflow.runtime.user_context import get_effective_user_id
 
 
 def resolve_thread_virtual_path(thread_id: str, virtual_path: str) -> Path:
@@ -22,7 +23,7 @@ def resolve_thread_virtual_path(thread_id: str, virtual_path: str) -> Path:
         HTTPException: If the path is invalid or outside allowed directories.
     """
     try:
-        return get_paths().resolve_virtual_path(thread_id, virtual_path)
+        return get_paths().resolve_virtual_path(thread_id, virtual_path, user_id=get_effective_user_id())
     except ValueError as e:
         status = 403 if "traversal" in str(e) else 400
         raise HTTPException(status_code=status, detail=str(e))