refactor(agents): move checkpointer + mention_resolver to app/agents/shared (slice 5b)

Two independent leaf modules (no intra-new_chat deps, no frozen importer),
consumed only by flows/routes/tests. Flipped 8 importers across both the
dotted-path and module-style (from app.agents.new_chat import mention_resolver)
forms. No shims needed.

surfsense_backend/app/tasks/chat/stream_new_chat.py

@@ -28,7 +28,7 @@ from sqlalchemy.future import select
 
 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.new_chat.checkpointer import get_checkpointer
+from app.agents.shared.checkpointer import get_checkpointer
 from app.agents.shared.context import SurfSenseContextSchema
 from app.agents.shared.errors import BusyError
 from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
@@ -39,7 +39,7 @@ from app.agents.shared.llm_config import (
     load_agent_config,
     load_global_llm_config_by_id,
 )
-from app.agents.new_chat.mention_resolver import resolve_mentions, substitute_in_text
+from app.agents.shared.mention_resolver import resolve_mentions, substitute_in_text
 from app.agents.new_chat.middleware.busy_mutex import (
     end_turn,
     get_cancel_state,

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

@@ -29,7 +29,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.future import select
 
 from app.agents.shared.filesystem_selection import FilesystemMode
-from app.agents.new_chat.mention_resolver import resolve_mentions, substitute_in_text
+from app.agents.shared.mention_resolver import resolve_mentions, substitute_in_text
 from app.db import (
     ChatVisibility,
     NewChatThread,

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

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.agents.new_chat.checkpointer import get_checkpointer
+from app.agents.shared.checkpointer import get_checkpointer
 from app.db import SearchSourceConnectorType
 from app.services.connector_service import ConnectorService
 
@@ -33,7 +33,7 @@ async def setup_connector_and_firecrawl(
 async def get_chat_checkpointer():
     """Resolve the PostgreSQL checkpointer for persistent conversation memory.
 
-    Thin wrapper around ``app.agents.new_chat.checkpointer.get_checkpointer`` so
+    Thin wrapper around ``app.agents.shared.checkpointer.get_checkpointer`` so
     flow orchestrators can rely on a streaming-local symbol and we have a hook
     point if the checkpointer source ever needs to vary per flow.
     """