refactor(agents): colocate main-agent middleware under main_agent/ slice

Vertical-slice colocation: all main-agent code should live under
main_agent/ instead of being split across a parallel middleware/main_agent
tree. Move multi_agent_chat/middleware/main_agent/ -> main_agent/middleware/
and its assembler middleware/stack.py -> main_agent/middleware/stack.py, so
the main-agent slice is self-contained (graph, runtime, system_prompt, tools,
middleware).

Genuinely cross-slice middleware (middleware/shared/, middleware/subagent/)
stays under multi_agent_chat/middleware/ for a later slice; the moved builders
now reference it via absolute imports.

Pure move + import rewrite (git-tracked renames). Verified: full unit suite
green (2430 passed, 1 skipped), including test_import_all and the
checkpointed-subagent middleware suite.

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_hitl_bridge.py

@@ -14,14 +14,14 @@ from langgraph.graph import END, START, StateGraph
 from langgraph.types import Command, interrupt
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.config import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.config import (
     subagent_invoke_config,
 )
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.resume_routing import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.resume_routing import (
     collect_pending_tool_calls,
     slice_decisions_by_tool_call,
 )
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.task_tool import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
 

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_parallel_heterogeneous_decisions.py

@@ -40,12 +40,12 @@ from langgraph.graph.message import add_messages
 from langgraph.types import Command, Send, interrupt
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.resume_routing import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.resume_routing import (
     build_lg_resume_map,
     collect_pending_tool_calls,
     slice_decisions_by_tool_call,
 )
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.task_tool import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
 

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_parallel_partial_pause_routing.py

@@ -47,12 +47,12 @@ from langgraph.graph.message import add_messages
 from langgraph.types import Command, Send, interrupt
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.resume_routing import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.resume_routing import (
     build_lg_resume_map,
     collect_pending_tool_calls,
     slice_decisions_by_tool_call,
 )
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.task_tool import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
 

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_parallel_reject_only_routing.py

@@ -37,12 +37,12 @@ from langgraph.graph.message import add_messages
 from langgraph.types import Command, Send, interrupt
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.resume_routing import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.resume_routing import (
     build_lg_resume_map,
     collect_pending_tool_calls,
     slice_decisions_by_tool_call,
 )
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.task_tool import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
 

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_parallel_resume_command_keying.py

@@ -37,12 +37,12 @@ from langgraph.graph.message import add_messages
 from langgraph.types import Command, Send, interrupt
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.resume_routing import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.resume_routing import (
     build_lg_resume_map,
     collect_pending_tool_calls,
     slice_decisions_by_tool_call,
 )
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.task_tool import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
 

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_parallel_self_and_middleware_gated.py

@@ -35,12 +35,12 @@ from langgraph.graph.message import add_messages
 from langgraph.types import Command, Send
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.resume_routing import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.resume_routing import (
     build_lg_resume_map,
     collect_pending_tool_calls,
     slice_decisions_by_tool_call,
 )
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.task_tool import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
 from app.agents.multi_agent_chat.middleware.shared.permissions.ask.request import (

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_parallel_tasks.py

@@ -18,7 +18,7 @@ from langgraph.graph import END, START, StateGraph
 from langgraph.types import Command
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.task_tool import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
 

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_pending_interrupt.py

@@ -9,7 +9,7 @@ from __future__ import annotations
 
 from types import SimpleNamespace
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.resume import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.resume import (
     get_first_pending_subagent_interrupt,
 )
 

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_resume_decision_routing.py

@@ -17,7 +17,7 @@ from types import SimpleNamespace
 
 import pytest
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.resume_routing import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.resume_routing import (
     collect_pending_tool_calls,
     slice_decisions_by_tool_call,
 )

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_resume_helpers.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 from langchain.tools import ToolRuntime
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.config import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.config import (
     consume_surfsense_resume,
     has_surfsense_resume,
 )

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_subagent_interrupt_stamping.py

@@ -30,7 +30,7 @@ from langgraph.graph import END, START, StateGraph
 from langgraph.types import Send, interrupt
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.task_tool import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
 

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_subagent_invoke_config.py

@@ -16,7 +16,7 @@ from __future__ import annotations
 
 from langchain.tools import ToolRuntime
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.config import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.config import (
     subagent_invoke_config,
 )
 

surfsense_backend/tests/unit/tasks/chat/streaming/test_interrupt_inspector_all.py

@@ -18,7 +18,7 @@ from langgraph.graph import END, START, StateGraph
 from langgraph.types import Send, interrupt
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.main_agent.checkpointed_subagent_middleware.task_tool import (
+from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
 from app.tasks.chat.streaming.helpers.interrupt_inspector import (