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

memory (builder) + memory_injection (impl) lived in shared/middleware/ but are
consumed only by main_agent (no subagent, no shared plumbing). Colocate with
their owner using the folder-per-middleware shape:

  shared/middleware/memory.py            -> main_agent/middleware/memory/builder.py
  shared/middleware/memory_injection.py  -> main_agent/middleware/memory/middleware.py

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

@@ -0,0 +1,5 @@
+"""User/team memory injection middleware (main-agent only)."""
+
+from .builder import build_memory_mw
+
+__all__ = ["build_memory_mw"]

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

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 from app.db import ChatVisibility
 
-from .memory_injection import MemoryInjectionMiddleware
+from .middleware import MemoryInjectionMiddleware
 
 
 def build_memory_mw(

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

@@ -20,6 +20,9 @@ from langchain_core.language_models import BaseChatModel
 from langchain_core.tools import BaseTool
 from langgraph.types import Checkpointer
 
+from app.agents.chat.multi_agent_chat.main_agent.middleware.memory import (
+    build_memory_mw,
+)
 from app.agents.chat.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
 from app.agents.chat.multi_agent_chat.shared.filesystem_selection import FilesystemMode
 from app.agents.chat.multi_agent_chat.shared.middleware.anthropic_cache import (
@@ -31,7 +34,6 @@ from app.agents.chat.multi_agent_chat.shared.middleware.compaction import (
 from app.agents.chat.multi_agent_chat.shared.middleware.kb_context_projection import (
     build_kb_context_projection_mw,
 )
-from app.agents.chat.multi_agent_chat.shared.middleware.memory import build_memory_mw
 from app.agents.chat.multi_agent_chat.shared.middleware.patch_tool_calls import (
     build_patch_tool_calls_mw,
 )