refactor(agents): move kb_persistence middleware into main_agent (owner)

The KB-persistence impl lived in shared/middleware/ but no subagent uses it --
consumers are the main_agent builder and the boundary event loop. Colocate with
its owner using the folder-per-middleware shape; __init__ re-exports the public
surface. Tests that reached module internals now alias the .middleware submodule.

  main_agent/middleware/kb_persistence.py -> kb_persistence/builder.py
  shared/middleware/kb_persistence.py     -> kb_persistence/middleware.py

surfsense_backend/app/agents/chat/multi_agent_chat/main_agent/middleware/kb_persistence/__init__.py

@@ -0,0 +1,13 @@
+"""End-of-turn KB persistence middleware (main-agent only)."""
+
+from .builder import build_kb_persistence_mw
+from .middleware import (
+    KnowledgeBasePersistenceMiddleware,
+    commit_staged_filesystem_state,
+)
+
+__all__ = [
+    "KnowledgeBasePersistenceMiddleware",
+    "build_kb_persistence_mw",
+    "commit_staged_filesystem_state",
+]

surfsense_backend/app/agents/chat/multi_agent_chat/main_agent/middleware/kb_persistence/builder.py

@@ -3,7 +3,8 @@
 from __future__ import annotations
 
 from app.agents.chat.multi_agent_chat.shared.filesystem_selection import FilesystemMode
-from app.agents.chat.multi_agent_chat.shared.middleware.kb_persistence import (
+
+from .middleware import (
     KnowledgeBasePersistenceMiddleware,
 )
 

surfsense_backend/app/tasks/chat/streaming/agent/event_loop.py

@@ -11,10 +11,10 @@ from __future__ import annotations
 from collections.abc import AsyncGenerator
 from typing import Any
 
-from app.agents.chat.multi_agent_chat.shared.filesystem_selection import FilesystemMode
+from app.agents.chat.multi_agent_chat.main_agent.middleware.kb_persistence import (
-from app.agents.chat.multi_agent_chat.shared.middleware.kb_persistence import (
     commit_staged_filesystem_state,
 )
+from app.agents.chat.multi_agent_chat.shared.filesystem_selection import FilesystemMode
 from app.services.new_streaming_service import VercelStreamingService
 from app.tasks.chat.streaming.contract.file_contract import (
     contract_enforcement_active,

surfsense_backend/tests/unit/middleware/test_kb_persistence_filesystem_parity.py

@@ -15,7 +15,7 @@ from unittest.mock import AsyncMock
 import numpy as np
 import pytest
 
-from app.agents.chat.multi_agent_chat.shared.middleware import kb_persistence
+from app.agents.chat.multi_agent_chat.main_agent.middleware.kb_persistence import middleware as kb_persistence
 from app.db import Document
 
 

surfsense_backend/tests/unit/middleware/test_kb_persistence_revisions.py

@@ -21,7 +21,7 @@ from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 
-from app.agents.chat.multi_agent_chat.shared.middleware import kb_persistence
+from app.agents.chat.multi_agent_chat.main_agent.middleware.kb_persistence import middleware as kb_persistence
 
 pytestmark = pytest.mark.unit