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/tests/integration/retriever/test_knowledge_search_date_filters.py

@@ -8,7 +8,7 @@ from datetime import UTC, datetime, timedelta
 import numpy as np
 import pytest
 
-from app.agents.new_chat.middleware.knowledge_search import search_knowledge_base
+from app.agents.shared.middleware.knowledge_search import search_knowledge_base
 
 from .conftest import DUMMY_EMBEDDING
 
@@ -27,11 +27,11 @@ async def test_search_knowledge_base_applies_date_filters(
         yield db_session
 
     monkeypatch.setattr(
-        "app.agents.new_chat.middleware.knowledge_search.shielded_async_session",
+        "app.agents.shared.middleware.knowledge_search.shielded_async_session",
         fake_shielded_async_session,
     )
     monkeypatch.setattr(
-        "app.agents.new_chat.middleware.knowledge_search.embed_texts",
+        "app.agents.shared.middleware.knowledge_search.embed_texts",
         lambda texts: [np.array(DUMMY_EMBEDDING) for _ in texts],
     )