refactor(agents): colocate middleware into vertical slices

Eliminate the top-level multi_agent_chat/middleware/ package so each slice
owns its middleware (vertical-slice colocation):

- middleware/shared/   -> shared/middleware/        (cross-slice middleware)
- middleware/subagent/ -> subagents/shared/middleware/ (subagent stack)
- main_agent/middleware/ already colocated in Slice A

The moved shared/ subtree is internally consistent (all relative imports
stay within it), so only external absolute refs were rewritten. The
subagent stack's ..shared.* relatives were promoted to absolute paths to
the new shared/middleware/ location.

multi_agent_chat/ root is now: main_agent/, shared/, subagents/.
Verified: 2430 unit tests pass, 1 skipped (baseline unchanged).

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

@@ -43,7 +43,7 @@ from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_mid
 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 (
+from app.agents.multi_agent_chat.shared.middleware.permissions.ask.request import (
     request_permission_decision,
 )
 from app.agents.multi_agent_chat.subagents.shared.hitl.approvals.self_gated import (

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_lc_hitl_wire.py

@@ -16,7 +16,7 @@ from langgraph.graph import END, START, StateGraph
 from langgraph.types import Command
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.shared.permissions.ask.request import (
+from app.agents.multi_agent_chat.shared.middleware.permissions.ask.request import (
     request_permission_decision,
 )
 from app.agents.shared.permissions import Rule

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_permission_ask_mcp_context.py

@@ -13,10 +13,10 @@ from langgraph.graph.message import add_messages
 from pydantic import BaseModel
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.shared.permissions import (
+from app.agents.multi_agent_chat.shared.middleware.permissions import (
     build_permission_mw,
 )
-from app.agents.multi_agent_chat.middleware.shared.permissions.ask.payload import (
+from app.agents.multi_agent_chat.shared.middleware.permissions.ask.payload import (
     build_permission_ask_payload,
 )
 from app.agents.shared.feature_flags import AgentFeatureFlags

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_subagent_owned_ruleset.py

@@ -23,7 +23,7 @@ from langgraph.graph.message import add_messages
 from langgraph.types import Command
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.shared.permissions import (
+from app.agents.multi_agent_chat.shared.middleware.permissions import (
     build_permission_mw,
 )
 from app.agents.shared.feature_flags import AgentFeatureFlags

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_trusted_tool_save_on_always.py

@@ -14,7 +14,7 @@ from langgraph.types import Command
 from pydantic import BaseModel
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.middleware.shared.permissions import (
+from app.agents.multi_agent_chat.shared.middleware.permissions import (
     build_permission_mw,
 )
 from app.agents.shared.feature_flags import AgentFeatureFlags

surfsense_backend/tests/unit/agents/multi_agent_chat/subagents/shared/test_subagent_builder.py

@@ -19,7 +19,7 @@ from langchain_core.language_models.fake_chat_models import (
 from langchain_core.messages import AIMessage, BaseMessage, HumanMessage
 from langchain_core.outputs import ChatGeneration, ChatResult
 
-from app.agents.multi_agent_chat.middleware.shared.permissions.middleware.core import (
+from app.agents.multi_agent_chat.shared.middleware.permissions.middleware.core import (
     PermissionMiddleware,
 )
 from app.agents.multi_agent_chat.subagents.shared.subagent_builder import (