The responder loads the issuer and callout-user seeds from the secret
store under its own role, beside the xkey, and no longer takes
--issuer-seed-file / --user-seed-file.
`ensure-xkey` becomes `ensure-callout-keys`: it ensures the xkey as before,
and for each nkey creates the seed create-only when the store holds none,
from --import-{issuer,user}-seed-file when that file is present, so a key
kept on disk keeps its public half. Once a read of the store returns a seed,
the import file is removed; when that seed differs from the file the
store's is kept and a warning names the file. The public halves are
published create-only at swarm/public/swarm-nats/callout-{issuer,user}.
The KeyStore trait and its test double move to `keystore` so both key
modules share them.
