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

The busy-mutex impl (BusyMutexMiddleware + cancel/turn-lifecycle primitives)
lived in shared/middleware/ but no subagent uses it -- consumers are the
main_agent builder and the boundary (turn lifecycle). Colocate with its owner
using the folder-per-middleware shape; __init__ re-exports the public surface so
boundary import sites only change package path:

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

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

@@ -0,0 +1,25 @@
+"""Per-turn cooperative busy-lock middleware + cancel primitives (main-agent)."""
+
+from .builder import build_busy_mutex_mw
+from .middleware import (
+    BusyMutexMiddleware,
+    end_turn,
+    get_cancel_event,
+    get_cancel_state,
+    is_cancel_requested,
+    manager,
+    request_cancel,
+    reset_cancel,
+)
+
+__all__ = [
+    "BusyMutexMiddleware",
+    "build_busy_mutex_mw",
+    "end_turn",
+    "get_cancel_event",
+    "get_cancel_state",
+    "is_cancel_requested",
+    "manager",
+    "request_cancel",
+    "reset_cancel",
+]

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

@@ -3,10 +3,11 @@
 from __future__ import annotations
 
 from app.agents.chat.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
-from app.agents.chat.multi_agent_chat.shared.middleware.busy_mutex import (
+from app.agents.chat.multi_agent_chat.shared.middleware.flags import enabled
+
+from .middleware import (
     BusyMutexMiddleware,
 )
-from app.agents.chat.multi_agent_chat.shared.middleware.flags import enabled
 
 
 def build_busy_mutex_mw(flags: AgentFeatureFlags) -> BusyMutexMiddleware | None:

surfsense_backend/app/routes/new_chat_routes.py

@@ -24,18 +24,18 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.future import select
 from sqlalchemy.orm import selectinload
 
+from app.agents.chat.multi_agent_chat.main_agent.middleware.busy_mutex import (
+    get_cancel_state,
+    is_cancel_requested,
+    manager,
+    request_cancel,
+)
 from app.agents.chat.multi_agent_chat.shared.filesystem_selection import (
     ClientPlatform,
     FilesystemMode,
     FilesystemSelection,
     LocalFilesystemMount,
 )
-from app.agents.chat.multi_agent_chat.shared.middleware.busy_mutex import (
-    get_cancel_state,
-    is_cancel_requested,
-    manager,
-    request_cancel,
-)
 from app.config import config
 from app.db import (
     ChatComment,

surfsense_backend/app/tasks/chat/streaming/errors/classifier.py

@@ -7,7 +7,7 @@ import logging
 import time
 from typing import Any, Literal
 
-from app.agents.chat.multi_agent_chat.shared.middleware.busy_mutex import (
+from app.agents.chat.multi_agent_chat.main_agent.middleware.busy_mutex import (
     get_cancel_state,
     is_cancel_requested,
 )

surfsense_backend/app/tasks/chat/streaming/flows/new_chat/orchestrator.py

@@ -30,11 +30,11 @@ from typing import Any, Literal
 import anyio
 
 from app.agents.chat.multi_agent_chat import create_multi_agent_chat_deep_agent
+from app.agents.chat.multi_agent_chat.main_agent.middleware.busy_mutex import end_turn
 from app.agents.chat.multi_agent_chat.shared.filesystem_selection import (
     FilesystemMode,
     FilesystemSelection,
 )
-from app.agents.chat.multi_agent_chat.shared.middleware.busy_mutex import end_turn
 from app.db import ChatVisibility, async_session_maker
 from app.observability import otel as ot
 from app.services.new_streaming_service import VercelStreamingService

surfsense_backend/app/tasks/chat/streaming/flows/resume_chat/orchestrator.py

@@ -24,11 +24,11 @@ from uuid import UUID
 import anyio
 
 from app.agents.chat.multi_agent_chat import create_multi_agent_chat_deep_agent
+from app.agents.chat.multi_agent_chat.main_agent.middleware.busy_mutex import end_turn
 from app.agents.chat.multi_agent_chat.shared.filesystem_selection import (
     FilesystemMode,
     FilesystemSelection,
 )
-from app.agents.chat.multi_agent_chat.shared.middleware.busy_mutex import end_turn
 from app.db import ChatVisibility, async_session_maker
 from app.observability import otel as ot
 from app.services.chat_session_state_service import set_ai_responding

surfsense_backend/app/tasks/chat/streaming/flows/shared/rate_limit_recovery.py

@@ -17,7 +17,7 @@ from typing import Literal
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.agents.chat.multi_agent_chat.shared.middleware.busy_mutex import end_turn
+from app.agents.chat.multi_agent_chat.main_agent.middleware.busy_mutex import end_turn
 from app.observability import otel as ot
 from app.services.auto_model_pin_service import (
     mark_runtime_cooldown,