refactor(agents): group filesystem backends under filesystem/backends/

The concrete filesystem backends are consumed only by the MAC filesystem
layer (tools, path-resolution middleware, the resolver, skills backend) and
tests -- no external app code. Group them next to the filesystem middleware
they serve:

- filesystem_backends.py            -> filesystem/backends/resolver.py
- middleware/kb_postgres_backend.py -> filesystem/backends/kb_postgres.py
- middleware/local_folder_backend.py -> filesystem/backends/local_folder.py
- middleware/multi_root_local_folder_backend.py -> .../multi_root_local_folder.py
- document_xml.py                   -> filesystem/backends/document_xml.py

Repoint all 21 importers. No behavior change; import-all + filesystem
backend/path-resolution/knowledge-search unit tests stay green (478).

surfsense_backend/tests/unit/middleware/test_b_filesystem_path_resolution.py

@@ -20,6 +20,9 @@ from types import SimpleNamespace
 
 import pytest
 
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.multi_root_local_folder import (
+    MultiRootLocalFolderBackend,
+)
 from app.agents.multi_agent_chat.shared.middleware.filesystem.middleware.mode import (
     default_cwd,
 )
@@ -33,9 +36,6 @@ from app.agents.multi_agent_chat.shared.middleware.filesystem.middleware.path_re
     resolve_relative,
 )
 from app.agents.shared.filesystem_selection import FilesystemMode
-from app.agents.shared.middleware.multi_root_local_folder_backend import (
-    MultiRootLocalFolderBackend,
-)
 
 pytestmark = pytest.mark.unit
 

surfsense_backend/tests/unit/middleware/test_b_filesystem_rm_rmdir_cloud.py

@@ -22,10 +22,14 @@ import pytest
 from app.agents.multi_agent_chat.shared.middleware.filesystem import (
     build_filesystem_mw,
 )
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.kb_postgres import (
+    KBPostgresBackend,
+)
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.resolver import (
+    build_backend_resolver,
+)
 from app.agents.multi_agent_chat.shared.state.reducers import _CLEAR
-from app.agents.shared.filesystem_backends import build_backend_resolver
 from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
-from app.agents.shared.middleware.kb_postgres_backend import KBPostgresBackend
 
 pytestmark = pytest.mark.unit
 

surfsense_backend/tests/unit/middleware/test_filesystem_backends.py

@@ -2,16 +2,18 @@ from pathlib import Path
 
 import pytest
 
-from app.agents.shared.filesystem_backends import build_backend_resolver
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.multi_root_local_folder import (
+    MultiRootLocalFolderBackend,
+)
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.resolver import (
+    build_backend_resolver,
+)
 from app.agents.shared.filesystem_selection import (
     ClientPlatform,
     FilesystemMode,
     FilesystemSelection,
     LocalFilesystemMount,
 )
-from app.agents.shared.middleware.multi_root_local_folder_backend import (
-    MultiRootLocalFolderBackend,
-)
 
 pytestmark = pytest.mark.unit
 

surfsense_backend/tests/unit/middleware/test_knowledge_search.py

@@ -5,7 +5,9 @@ import json
 import pytest
 from langchain_core.messages import AIMessage, HumanMessage
 
-from app.agents.shared.document_xml import build_document_xml as _build_document_xml
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.document_xml import (
+    build_document_xml as _build_document_xml,
+)
 from app.agents.shared.middleware.knowledge_search import (
     KBSearchPlan,
     KnowledgeBaseSearchMiddleware,

surfsense_backend/tests/unit/middleware/test_local_folder_backend.py

@@ -2,7 +2,9 @@ from pathlib import Path
 
 import pytest
 
-from app.agents.shared.middleware.local_folder_backend import LocalFolderBackend
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.local_folder import (
+    LocalFolderBackend,
+)
 
 pytestmark = pytest.mark.unit
 

surfsense_backend/tests/unit/middleware/test_multi_root_local_folder_backend.py

@@ -2,7 +2,7 @@ from pathlib import Path
 
 import pytest
 
-from app.agents.shared.middleware.multi_root_local_folder_backend import (
+from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.multi_root_local_folder import (
     MultiRootLocalFolderBackend,
 )