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: