refactor(multi-agent): add subagent extras builder and drop filesystem from registry subagents

surfsense_backend/app/agents/multi_agent_chat/middleware/subagent/extras.py

@@ -0,0 +1,28 @@
+"""Extra middleware threaded into every registry subagent's stack.
+
+Registry subagents are scoped to one domain (deliverables, research, memory,
+connectors, MCP) and never read or write the SurfSense filesystem — that
+capability belongs to the main agent and is delegated to the general-purpose
+subagent as an escape hatch. Keeping FS off the registry stacks avoids
+polluting their tool surface with FS tools they never act on.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from ..shared.permissions import PermissionContext
+from ..shared.resilience import ResilienceBundle
+from ..shared.todos import build_todos_mw
+
+
+def build_subagent_extras(
+    *,
+    permissions: PermissionContext,
+    resilience: ResilienceBundle,
+) -> list[Any]:
+    extras: list[Any] = [build_todos_mw()]
+    if permissions.subagent_deny_mw is not None:
+        extras.append(permissions.subagent_deny_mw)
+    extras.extend(resilience.as_list())
+    return extras