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".

hive-agent/src/main.rs

@@ -28,6 +28,7 @@ mod serve_common;
 mod state_entry_watch;
 mod stats;
 mod stream_enrich;
+mod swarm_agent_icon;
 mod swarm_agent_state;
 mod swarm_queue;
 mod swarm_term;
@@ -521,6 +522,8 @@ async fn serve_main<S: Surface>(socket: &Path, poll_ms: u64) -> Result<()> {
     // state from this agent's first transition, not from whenever a reader
     // first asked.
     swarm_agent_state::spawn(&bus);
+    // Once per start: a new icon only arrives with a new harness process.
+    swarm_agent_icon::spawn();
     // Set by the web UI's `/api/cancel` on a successful SIGINT, read-and-
     // cleared by `handle_turn` before building the next wake prompt — see
     // `hive_sh4re::inbox::INTERRUPTED_HINT`. Shared between the web server task and