swarm: let an agent publish its own icon

The auth callout grants an agent that presents its own queue credential
one more subject, `$KV.agent-icons.<agent>`: its own key in the
agent-icons bucket and no other. The hive's shared agent client is
granted none of the bucket, since every agent on a hive presents it.

hive-agent writes `/etc/hyperhive/icon.svg`, the file its `GET /icon`
serves, to that key once per start, as a JetStream publish straight to
the subject (what `kv::Store::put` sends, minus the bucket lookup), so
the one subject is the whole grant. No icon deletes the key. A failed
write, including one that arrives before the bucket exists, is retried
with backoff until acked. An agent connected with the hive's shared
client publishes nothing.

swarm-controller creates the bucket as soon as its queue connection is
up, instead of on the first icon read, so an agent's write does not
wait for someone to look.

Measured against a local nats-server with a user allowed publish on
`$KV.agent-icons.atlas` only: the write to its own key is stored and
readable, a write to `$KV.agent-icons.argus` is refused (the ack times
out), the DEL marker makes the key read as absent, and a write before
the bucket exists fails with "no responders".

swarm-queue-client/src/lib.rs

@@ -184,8 +184,7 @@ pub mod agent_token;
 
 /// The bucket agent icons are published into — one key per agent, with no
 /// hive in it, unlike [`agent_status`]. See the module doc for why the
-/// placement stays out of the key, and for what still has to land before
-/// anything can write it.
+/// placement stays out of the key, and for who may write it.
 pub mod agent_icon;
 
 /// The subject the swarm controller publishes on when the hive-wide knowledge