Completes slice 5. filesystem_backends was deferred from 3b because it depends
on middleware.{kb_postgres_backend,multi_root_local_folder_backend}; those moved
to shared in 5c, so it now relocates cleanly. Flip the 2 non-frozen importers
(multi-agent factory + test); a re-export shim remains for the frozen
chat_deepagent (build_backend_resolver).
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).
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.
