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/app/agents/multi_agent_chat/main_agent/middleware/action_log.py

@@ -8,7 +8,7 @@ from app.agents.shared.feature_flags import AgentFeatureFlags
 from app.agents.shared.middleware import ActionLogMiddleware
 from app.agents.shared.tools.registry import BUILTIN_TOOLS
 
-from app.agents.multi_agent_chat.middleware.shared.flags import enabled
+from app.agents.multi_agent_chat.shared.middleware.flags import enabled
 
 
 def build_action_log_mw(

surfsense_backend/app/agents/multi_agent_chat/main_agent/middleware/busy_mutex.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 from app.agents.shared.feature_flags import AgentFeatureFlags
 from app.agents.shared.middleware import BusyMutexMiddleware
 
-from app.agents.multi_agent_chat.middleware.shared.flags import enabled
+from app.agents.multi_agent_chat.shared.middleware.flags import enabled
 
 
 def build_busy_mutex_mw(flags: AgentFeatureFlags) -> BusyMutexMiddleware | None:

surfsense_backend/app/agents/multi_agent_chat/main_agent/middleware/context_editing.py

@@ -17,7 +17,7 @@ from app.agents.shared.middleware import (
     SpillToBackendEdit,
 )
 
-from app.agents.multi_agent_chat.middleware.shared.flags import enabled
+from app.agents.multi_agent_chat.shared.middleware.flags import enabled
 
 
 def build_context_editing_mw(

surfsense_backend/app/agents/multi_agent_chat/main_agent/middleware/doom_loop.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 from app.agents.shared.feature_flags import AgentFeatureFlags
 from app.agents.shared.middleware import DoomLoopMiddleware
 
-from app.agents.multi_agent_chat.middleware.shared.flags import enabled
+from app.agents.multi_agent_chat.shared.middleware.flags import enabled
 
 
 def build_doom_loop_mw(flags: AgentFeatureFlags) -> DoomLoopMiddleware | None:

surfsense_backend/app/agents/multi_agent_chat/main_agent/middleware/noop_injection.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 from app.agents.shared.feature_flags import AgentFeatureFlags
 from app.agents.shared.middleware import NoopInjectionMiddleware
 
-from app.agents.multi_agent_chat.middleware.shared.flags import enabled
+from app.agents.multi_agent_chat.shared.middleware.flags import enabled
 
 
 def build_noop_injection_mw(flags: AgentFeatureFlags) -> NoopInjectionMiddleware | None:

surfsense_backend/app/agents/multi_agent_chat/main_agent/middleware/otel.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 from app.agents.shared.feature_flags import AgentFeatureFlags
 from app.agents.shared.middleware import OtelSpanMiddleware
 
-from app.agents.multi_agent_chat.middleware.shared.flags import enabled
+from app.agents.multi_agent_chat.shared.middleware.flags import enabled
 
 
 def build_otel_mw(flags: AgentFeatureFlags) -> OtelSpanMiddleware | None:

surfsense_backend/app/agents/multi_agent_chat/main_agent/middleware/plugins.py

@@ -15,7 +15,7 @@ from app.agents.shared.plugin_loader import (
 )
 from app.db import ChatVisibility
 
-from app.agents.multi_agent_chat.middleware.shared.flags import enabled
+from app.agents.multi_agent_chat.shared.middleware.flags import enabled
 
 
 def build_plugin_middlewares(

surfsense_backend/app/agents/multi_agent_chat/main_agent/middleware/repair.py

@@ -9,7 +9,7 @@ from langchain_core.tools import BaseTool
 from app.agents.shared.feature_flags import AgentFeatureFlags
 from app.agents.shared.middleware import ToolCallNameRepairMiddleware
 
-from app.agents.multi_agent_chat.middleware.shared.flags import enabled
+from app.agents.multi_agent_chat.shared.middleware.flags import enabled
 
 # deepagents-built-in tool names the repair pass treats as known.
 _DEEPAGENT_BUILTIN_TOOL_NAMES: frozenset[str] = frozenset(

surfsense_backend/app/agents/multi_agent_chat/main_agent/middleware/skills.py

@@ -13,7 +13,7 @@ from app.agents.shared.middleware import (
     default_skills_sources,
 )
 
-from app.agents.multi_agent_chat.middleware.shared.flags import enabled
+from app.agents.multi_agent_chat.shared.middleware.flags import enabled
 
 
 def build_skills_mw(

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

@@ -55,15 +55,15 @@ from .otel import build_otel_mw
 from .plugins import build_plugin_middlewares
 from .repair import build_repair_mw
 from .skills import build_skills_mw
-from app.agents.multi_agent_chat.middleware.shared.anthropic_cache import build_anthropic_cache_mw
-from app.agents.multi_agent_chat.middleware.shared.compaction import build_compaction_mw
-from app.agents.multi_agent_chat.middleware.shared.kb_context_projection import build_kb_context_projection_mw
-from app.agents.multi_agent_chat.middleware.shared.memory import build_memory_mw
-from app.agents.multi_agent_chat.middleware.shared.patch_tool_calls import build_patch_tool_calls_mw
-from app.agents.multi_agent_chat.middleware.shared.permissions import build_permission_mw
-from app.agents.multi_agent_chat.middleware.shared.resilience import build_resilience_middlewares
-from app.agents.multi_agent_chat.middleware.shared.todos import build_todos_mw
-from app.agents.multi_agent_chat.middleware.subagent.middleware_stack import (
+from app.agents.multi_agent_chat.shared.middleware.anthropic_cache import build_anthropic_cache_mw
+from app.agents.multi_agent_chat.shared.middleware.compaction import build_compaction_mw
+from app.agents.multi_agent_chat.shared.middleware.kb_context_projection import build_kb_context_projection_mw
+from app.agents.multi_agent_chat.shared.middleware.memory import build_memory_mw
+from app.agents.multi_agent_chat.shared.middleware.patch_tool_calls import build_patch_tool_calls_mw
+from app.agents.multi_agent_chat.shared.middleware.permissions import build_permission_mw
+from app.agents.multi_agent_chat.shared.middleware.resilience import build_resilience_middlewares
+from app.agents.multi_agent_chat.shared.middleware.todos import build_todos_mw
+from app.agents.multi_agent_chat.subagents.shared.middleware.middleware_stack import (
     build_subagent_middleware_stack,
 )