The auth-callout responder decides what an admitted client may publish
from two strings: the prefix marking a hive client, and the client id
allowed to read every hive's key. Both were literals in three places --
swarm-authelia.nix mints "hive-${name}", swarm-controller.nix defines
"swarm-controller", and the responder carried its own copies as clap
defaults because swarm-nats.nix passed neither.
Each producer now publishes its value as a readOnly option and the
responder's ExecStart reads them, so the agreement is one evaluation
rather than three strings that happen to be equal. Same pattern the
module already uses for `--account`, and the same argument
swarm-authelia.nix gives for publishing `machine` and `unit`.
Worth the change because the failure is silent and misattributed:
rename either principal and the responder starts denying the one that
stopped matching, a denial reaches a NATS client as a timeout rather
than an error, and a hive that is refused looks exactly like a hive
that has not reported yet.
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