refactor(agents): move mac-only modules out of the cross-agent shared kernel

app/agents/shared/ is a sibling of anonymous_chat/podcaster/multi_agent_chat/
video_presentation, so it should only hold code shared across 2+ of those
agents. In practice podcaster and video_presentation import nothing from it,
and anonymous_chat needs only context + compaction + retry_after + web_search.
Everything else was multi_agent_chat-only (the boundary just passes through).

Move the multi_agent_chat-only cluster into multi_agent_chat/shared/ (files
moved verbatim via git rename; ~116 import sites rewritten):

  errors, feature_flags, filesystem_selection, path_resolver, prompt_caching,
  sandbox, llm_config, mention_resolver
  middleware/busy_mutex, middleware/kb_persistence

busy_mutex/llm_config/mention_resolver are boundary-only but import the moved
modules, so they were folded in to avoid a backwards shared -> multi_agent_chat
dependency. main_agent builders now import the impls directly; the shared
middleware barrel keeps only the genuinely-shared compaction + retry_after.

Also delete the dead leftover shared/plugins and shared/skills dirs (live
copies already live under main_agent/).

Remaining in app/agents/shared/: context, system_prompt(+prompts), checkpointer,
middleware/{compaction,retry_after,dedup_tool_calls}, tools/. checkpointer and
system_prompt are boundary-only infra pending a dedicated home decision.

surfsense_backend/tests/unit/middleware/test_b_filesystem_path_resolution.py

@@ -20,6 +20,7 @@ from types import SimpleNamespace
 
 import pytest
 
+from app.agents.multi_agent_chat.shared.filesystem_selection import FilesystemMode
 from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.multi_root_local_folder import (
     MultiRootLocalFolderBackend,
 )
@@ -35,7 +36,6 @@ from app.agents.multi_agent_chat.shared.middleware.filesystem.middleware.path_re
     normalize_local_mount_path,
     resolve_relative,
 )
-from app.agents.shared.filesystem_selection import FilesystemMode
 
 pytestmark = pytest.mark.unit
 

surfsense_backend/tests/unit/middleware/test_b_filesystem_rm_rmdir_cloud.py

@@ -19,6 +19,10 @@ from unittest.mock import AsyncMock
 
 import pytest
 
+from app.agents.multi_agent_chat.shared.filesystem_selection import (
+    FilesystemMode,
+    FilesystemSelection,
+)
 from app.agents.multi_agent_chat.shared.middleware.filesystem import (
     build_filesystem_mw,
 )
@@ -29,7 +33,6 @@ from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.resolver
     build_backend_resolver,
 )
 from app.agents.multi_agent_chat.shared.state.reducers import _CLEAR
-from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
 
 pytestmark = pytest.mark.unit
 

surfsense_backend/tests/unit/middleware/test_b_filesystem_system_prompt.py

@@ -18,10 +18,10 @@ from __future__ import annotations
 
 import pytest
 
+from app.agents.multi_agent_chat.shared.filesystem_selection import FilesystemMode
 from app.agents.multi_agent_chat.shared.middleware.filesystem.system_prompt import (
     build_system_prompt,
 )
-from app.agents.shared.filesystem_selection import FilesystemMode
 
 pytestmark = pytest.mark.unit
 

surfsense_backend/tests/unit/middleware/test_filesystem_backends.py

@@ -2,18 +2,18 @@ from pathlib import Path
 
 import pytest
 
+from app.agents.multi_agent_chat.shared.filesystem_selection import (
+    ClientPlatform,
+    FilesystemMode,
+    FilesystemSelection,
+    LocalFilesystemMount,
+)
 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,
-)
 
 pytestmark = pytest.mark.unit
 

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.shared.middleware import kb_persistence
+from app.agents.multi_agent_chat.shared.middleware import 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.shared.middleware import kb_persistence
+from app.agents.multi_agent_chat.shared.middleware import kb_persistence
 
 pytestmark = pytest.mark.unit
 

surfsense_backend/tests/unit/middleware/test_knowledge_tree.py

@@ -12,7 +12,7 @@ from __future__ import annotations
 from app.agents.multi_agent_chat.main_agent.middleware.knowledge_tree.middleware import (
     KnowledgeTreeMiddleware,
 )
-from app.agents.shared.path_resolver import DOCUMENTS_ROOT
+from app.agents.multi_agent_chat.shared.path_resolver import DOCUMENTS_ROOT
 
 
 def _compute(folder_paths: list[str], doc_paths: list[str]) -> set[str]:
@@ -88,7 +88,7 @@ class TestFormatTreeRendering:
         folder_paths: list[str],
         doc_specs: list[dict],
     ) -> str:
-        from app.agents.shared.path_resolver import PathIndex
+        from app.agents.multi_agent_chat.shared.path_resolver import PathIndex
 
         index = PathIndex(
             folder_paths={i + 1: p for i, p in enumerate(folder_paths)},