No-op Accrue.Storage adapter — the v1.0 default (D6-04).
put/3echoes the key back untouched; no filesystem or network I/O.get/1returns{:error, :not_configured}.delete/1returns{:error, :not_configured}.
Hosts enable real storage by swapping :storage_adapter to a custom
module implementing Accrue.Storage. Accrue.Storage.Filesystem
lands in v1.1.