refactor(agents): move filesystem_selection to app/agents/shared (slice 3a)

Promote the filesystem mode contracts (FilesystemMode, FilesystemSelection,
ClientPlatform, LocalFilesystemMount) out of `new_chat` into the cross-agent
`app/agents/shared` kernel.

Pure leaf consumed across the whole multi-agent filesystem middleware/tool tree,
the chat flows/monolith, routes and tests. git mv (content unchanged) + flipped
all ~48 importers. A re-export shim remains at new_chat/filesystem_selection.py
only for the not-yet-retired single-agent (chat_deepagent).

Also updated the stream parity test's annotation normalizer to strip the new
app.agents.shared.filesystem_selection. prefix (the dataclasses' __module__
changed with the move), keeping monolith<->flows signature parity intact.

Behavior-preserving: only import paths change. 1326 tests green.

surfsense_backend/app/tasks/chat/stream_new_chat.py

@@ -31,7 +31,7 @@ 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.context import SurfSenseContextSchema
 from app.agents.shared.errors import BusyError
-from app.agents.new_chat.filesystem_selection import FilesystemMode, FilesystemSelection
+from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
 from app.agents.new_chat.llm_config import (
     AgentConfig,
     create_chat_litellm_from_agent_config,

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

@@ -9,7 +9,7 @@ from __future__ import annotations
 
 from typing import Any
 
-from app.agents.new_chat.filesystem_selection import FilesystemSelection
+from app.agents.shared.filesystem_selection import FilesystemSelection
 from app.agents.new_chat.llm_config import AgentConfig
 from app.db import ChatVisibility
 from app.services.connector_service import ConnectorService

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

@@ -11,7 +11,7 @@ from __future__ import annotations
 from collections.abc import AsyncGenerator
 from typing import Any
 
-from app.agents.new_chat.filesystem_selection import FilesystemMode
+from app.agents.shared.filesystem_selection import FilesystemMode
 from app.agents.new_chat.middleware.kb_persistence import (
     commit_staged_filesystem_state,
 )

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

@@ -28,7 +28,7 @@ from langchain_core.messages import HumanMessage
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.future import select
 
-from app.agents.new_chat.filesystem_selection import FilesystemMode
+from app.agents.shared.filesystem_selection import FilesystemMode
 from app.agents.new_chat.mention_resolver import resolve_mentions, substitute_in_text
 from app.db import (
     ChatVisibility,

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

@@ -31,7 +31,7 @@ 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.new_chat.filesystem_selection import FilesystemMode, FilesystemSelection
+from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
 from app.agents.new_chat.middleware.busy_mutex import end_turn
 from app.config import config as _app_config
 from app.db import ChatVisibility, async_session_maker

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

@@ -25,7 +25,7 @@ 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.new_chat.filesystem_selection import FilesystemMode, FilesystemSelection
+from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
 from app.agents.new_chat.middleware.busy_mutex import end_turn
 from app.config import config as _app_config
 from app.db import ChatVisibility, async_session_maker

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

@@ -15,7 +15,7 @@ from __future__ import annotations
 from collections.abc import AsyncGenerator, Awaitable, Callable
 from typing import Any
 
-from app.agents.new_chat.filesystem_selection import FilesystemMode
+from app.agents.shared.filesystem_selection import FilesystemMode
 from app.services.new_streaming_service import VercelStreamingService
 from app.tasks.chat.streaming.agent.event_loop import stream_agent_events
 from app.tasks.chat.streaming.shared.stream_result import StreamResult