refactor(agents): place subagent middleware-stack by primary-built-for

Re-decide subagent_stack placement using the primary-built-for lens rather
than consumer-only: it assembles the middleware stack threaded into every
subagent, so its domain is subagents -- even though main_agent is its sole
caller (analogous to subagents/registry.py, also invoked from main_agent).

Since no subagent *sibling* imports it, it does not belong in subagents/shared/
but at the subagents/ package root:

  main_agent/middleware/subagent_stack.py
  -> subagents/middleware_stack.py

surfsense_backend/app/agents/chat/multi_agent_chat/main_agent/middleware/stack.py

@@ -53,6 +53,9 @@ from app.agents.chat.multi_agent_chat.subagents.builtins.knowledge_base.agent im
 from app.agents.chat.multi_agent_chat.subagents.builtins.knowledge_base.ask_knowledge_base_tool import (
     build_ask_knowledge_base_tool,
 )
+from app.agents.chat.multi_agent_chat.subagents.middleware_stack import (
+    build_subagent_middleware_stack,
+)
 from app.db import ChatVisibility
 
 from .action_log import build_action_log_mw
@@ -74,9 +77,6 @@ from .noop_injection import build_noop_injection_mw
 from .otel_span import build_otel_mw
 from .plugins import build_plugin_middlewares
 from .skills import build_skills_mw
-from .subagent_stack import (
-    build_subagent_middleware_stack,
-)
 from .tool_call_repair import build_repair_mw