refactor(agents): move middleware package to app/agents/shared (slice 5c)

Relocate the entire new_chat/middleware/ package to the shared kernel as one
cohesive unit (it is live shared infrastructure: the multi-agent stack wraps
nearly every middleware via multi_agent_chat/middleware/main_agent/*, and
anonymous_agent consumes it too). Flip 69 live importers across both the
package-path and submodule-path forms.

Shims left for the frozen single-agent stack: a package __init__ re-export plus
submodule shims for permission, skills_backends, and scoped_model_fallback
(the three imported via submodule path by chat_deepagent/subagents).

Cycle break: importing shared.middleware previously reached back into
new_chat.tools at module load, which dragged in new_chat.__init__ ->
chat_deepagent -> the middleware shim -> half-initialized shared.middleware.
Made action_log's ToolDefinition import TYPE_CHECKING-only and
tool_call_repair's INVALID_TOOL_NAME import function-local. These tools-package
back-edges fully resolve in slice 6.

Asset note: skills_backends._default_builtin_root now walks to
app/agents/new_chat/skills/builtin (the skills/ tree migrates in slice 7).

surfsense_backend/app/tasks/chat/stream_new_chat.py

@@ -40,12 +40,12 @@ from app.agents.shared.llm_config import (
     load_global_llm_config_by_id,
 )
 from app.agents.shared.mention_resolver import resolve_mentions, substitute_in_text
-from app.agents.new_chat.middleware.busy_mutex import (
+from app.agents.shared.middleware.busy_mutex import (
     end_turn,
     get_cancel_state,
     is_cancel_requested,
 )
-from app.agents.new_chat.middleware.kb_persistence import (
+from app.agents.shared.middleware.kb_persistence import (
     commit_staged_filesystem_state,
 )
 from app.db import (

surfsense_backend/app/tasks/chat/streaming/agent/event_loop.py

@@ -12,7 +12,7 @@ from collections.abc import AsyncGenerator
 from typing import Any
 
 from app.agents.shared.filesystem_selection import FilesystemMode
-from app.agents.new_chat.middleware.kb_persistence import (
+from app.agents.shared.middleware.kb_persistence import (
     commit_staged_filesystem_state,
 )
 from app.services.new_streaming_service import VercelStreamingService

surfsense_backend/app/tasks/chat/streaming/errors/classifier.py

@@ -8,7 +8,7 @@ import time
 from typing import Any, Literal
 
 from app.agents.shared.errors import BusyError
-from app.agents.new_chat.middleware.busy_mutex import (
+from app.agents.shared.middleware.busy_mutex import (
     get_cancel_state,
     is_cancel_requested,
 )

surfsense_backend/app/tasks/chat/streaming/flows/new_chat/orchestrator.py

@@ -32,7 +32,7 @@ import anyio
 from app.agents.multi_agent_chat import create_multi_agent_chat_deep_agent
 from app.agents.new_chat.chat_deepagent import create_surfsense_deep_agent
 from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
-from app.agents.new_chat.middleware.busy_mutex import end_turn
+from app.agents.shared.middleware.busy_mutex import end_turn
 from app.config import config as _app_config
 from app.db import ChatVisibility, async_session_maker
 from app.observability import otel as ot

surfsense_backend/app/tasks/chat/streaming/flows/resume_chat/orchestrator.py

@@ -26,7 +26,7 @@ import anyio
 from app.agents.multi_agent_chat import create_multi_agent_chat_deep_agent
 from app.agents.new_chat.chat_deepagent import create_surfsense_deep_agent
 from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
-from app.agents.new_chat.middleware.busy_mutex import end_turn
+from app.agents.shared.middleware.busy_mutex import end_turn
 from app.config import config as _app_config
 from app.db import ChatVisibility, async_session_maker
 from app.observability import otel as ot

surfsense_backend/app/tasks/chat/streaming/flows/shared/rate_limit_recovery.py

@@ -17,7 +17,7 @@ from typing import Literal
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.agents.new_chat.middleware.busy_mutex import end_turn
+from app.agents.shared.middleware.busy_mutex import end_turn
 from app.observability import otel as ot
 from app.services.auto_model_pin_service import (
     mark_runtime_cooldown,