icons for the swarm's own forge accounts, and a default agent icon

The builder, ci-metrics and review-classifier forge accounts get an avatar: an
icon each in a hexagonal frame, the hyperhive logo when one is missing. The
controller sets them with a short-lived write:user token it mints for the
account, since only an account can set its own avatar.

An agent with no icon of its own shows a default agent icon in the swarm UI, a
robot head in a round frame, instead of the dimmed hyperhive mark.

nix/packages/default.nix

@@ -228,6 +228,7 @@ in
   swarm-ui = pkgs.callPackage ./swarm-ui.nix {
     branding-svg = ../../branding/hyperhive.svg;
     branding-svg-maskable = ../../branding/hyperhive-maskable.svg;
+    agent-default-svg = ../../branding/agent-default.svg;
   };
 
   # Bundled browser assets — see ./frontend.nix. Output is