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.
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