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/tests/unit/middleware/test_filesystem_backends.py

@@ -3,7 +3,7 @@ from pathlib import Path
 import pytest
 
 from app.agents.new_chat.filesystem_backends import build_backend_resolver
-from app.agents.new_chat.filesystem_selection import (
+from app.agents.shared.filesystem_selection import (
     ClientPlatform,
     FilesystemMode,
     FilesystemSelection,