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/README.md

@@ -19,8 +19,9 @@ simply owns the file it writes. No elevated privilege anywhere.
 ## Shape
 
 - One endpoint, `POST /requests`, body = `swarm-authelia-bridge-sock`'s
-  `BridgeRequest` verbatim — one variant today (`EnsureAgentIdentity`,
-  idempotently ensure an agent exists as an authelia subject).
+  `BridgeRequest` verbatim — `EnsureAgentIdentity` (idempotently ensure an
+  agent exists as an authelia subject) and `ListAgentIdentities` (the
+  roster: the subjects carrying the agent marker group, names only).
 - Bearer-authenticated via authelia's own OIDC token introspection (RFC
   7662), against `swarm-controller`'s **existing** machine-client identity
   (already minted for the queue connection) — no new credential.