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/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