feat(swarm-authelia-bridge): mark agent identities with a group, and answer for the set

The users database holds humans and agents in one namespace and nothing in
it said which was which, so a roster read had no predicate to read with.

Marks positively, at creation. The alternative — everyone who is not an
operator — fails in the direction that matters: an account created without
a group is an operator who cannot log in, a mistake the swarm UI docs
already warn about, and it would have rendered as an agent. The group is a
constant for the same reason the operator group it mirrors is one.

An identity that predates the marker gains it when agent creation runs
again, which is already the agreed migration for those; AlreadyExists
therefore reports that the subject was there, not that nothing was written.

ListAgentIdentities reads through this process because the store is owned
by a uid swarm-controller does not have — the same reason the write goes
through here — and answers with names alone, never the digests it sits next
to.

swarm-authelia-bridge-sock/README.md

@@ -16,8 +16,8 @@ client logic here, only the request/response shapes both sides import.
 
 ## Shape
 
-One operation today: idempotently ensure an agent exists as an authelia
-subject. Deliberately **not** a wholesale-replace-the-file API — the bridge
+Two operations: idempotently ensure an agent exists as an authelia subject,
+and list the ones that do. Deliberately **not** a wholesale-replace-the-file API — the bridge
 reads `users.yml`, changes what the request named, and writes it back; a
 caller only ever asks for one user to exist, never sends rendered YAML or a
 file blob. See `swarm-authelia-bridge/README.md` for the helper itself.