`swarm-secret-publisher` holds create and update on
`secret/data/swarm/services/*`, which covers the callout xkey's seed at
`swarm/services/swarm-nats/callout/xkey-seed`. It never writes that
path, but a write there would replace the key the queue's server seals
every callout request to. An exact-path `deny` outranks the prefix.
A create-only grant would not do: KV v2 honours create versus update, so
it would stop an overwrite, but it would still let the publisher plant a
seed of its choosing whenever the path is empty, on a first boot or
after a key replacement. The publisher needs no access to the path at
all.
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