offload_storage_is_ephemeral() -> boolReturn whether offload history is routed to non-persistent storage.
True when the persistent ~/.deepagents location was unwritable and the
most recent _offload_fallback_root fell back to a temporary directory that
may not survive a restart. Only meaningful in local mode, where
_offload_fallback_root runs in the same process as the UI; in
server/sandbox mode persistence is owned by the server backend and this flag
stays False client-side.