swarm: serve an agent's icon at swarm scope

An agent is not fixed to a hive, so its icon cannot be resolved as
hive -> agent. This adds the swarm-level half: an `agent-icons` KV
bucket keyed by the agent name alone — no hive token, so an agent that
moves hives keeps its icon and one that is stopped still has one — and
`GET /api/agents/<name>/icon` on swarm-controller serving it
same-origin, like every other `/api/*` route swarm-ui calls.

404 is the "this agent has no icon" answer, the same contract the
per-agent harness's own `GET /icon` has for an unconfigured agent.
Until the agent-side publisher lands, that is every agent's answer:
the publisher runs inside the container and an agent's NATS grants are
hive-scoped, which cannot authorise a write to a single-token agent
key. The read side needs no grant change — the controller already
holds `$KV.*.>` and `$JS.API.DIRECT.GET.*.>`.

The response carries `Content-Security-Policy: sandbox` and `nosniff`:
the body is an operator-authored SVG served from this daemon's own
origin, and an SVG can carry script.

Hive-side icon serving is untouched.

Refs #4502

swarm-controller/src/matrix_account.rs

@@ -573,6 +573,7 @@ mod tests {
             status: Some(std::sync::Arc::new(status)),
             wanted: None,
             agent_status: None,
+            agent_icons: None,
             jobq: std::sync::Arc::new(std::sync::Mutex::new(hive_jobq::scheduler::Scheduler::new(
                 hive_jobq::Graph::new(),
                 hive_jobq::resources::ResourceTable::new(),