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/integration/agents/multi_agent_chat/test_kb_filesystem_cloud.py

@@ -20,13 +20,16 @@ from langchain.agents import create_agent
 from langchain_core.messages import HumanMessage, ToolMessage
 from langgraph.checkpoint.memory import InMemorySaver
 
+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,
 )
 from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.resolver import (
     build_backend_resolver,
 )
-from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
 from tests.integration.harness import ScriptedTurn, build_scripted_harness
 
 pytestmark = [pytest.mark.integration, pytest.mark.asyncio]

surfsense_backend/tests/integration/agents/multi_agent_chat/test_kb_filesystem_desktop.py

@@ -23,17 +23,17 @@ from langchain.agents import create_agent
 from langchain_core.messages import HumanMessage, ToolMessage
 from langgraph.checkpoint.memory import InMemorySaver
 
+from app.agents.multi_agent_chat.shared.filesystem_selection import (
+    FilesystemMode,
+    FilesystemSelection,
+    LocalFilesystemMount,
+)
 from app.agents.multi_agent_chat.shared.middleware.filesystem import (
     build_filesystem_mw,
 )
 from app.agents.multi_agent_chat.shared.middleware.filesystem.backends.resolver import (
     build_backend_resolver,
 )
-from app.agents.shared.filesystem_selection import (
-    FilesystemMode,
-    FilesystemSelection,
-    LocalFilesystemMount,
-)
 from tests.integration.harness import ScriptedTurn, build_scripted_harness
 
 pytestmark = [pytest.mark.integration, pytest.mark.asyncio]