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/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,