A MintAgentForgeToken node mints a fixed-name swarm-agent token with the
admin API, keeps it when the stored value's last eight and the normalised
scopes match the forge's list, and otherwise deletes and re-creates it.
The token is stored at swarm/agents/<agent>/forge-token. Agent creation
inserts the node, and a pass at start and every five minutes inserts it
for every agent holding a store identity whose token is missing or stale.
Refs #3782
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