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).
