gateFunc, passthroughGate and isTransientExternal sat in agent_images.go
while every implementation lived in worker.go: extGate, breaker,
Worker.gate, gateFor. isTransientExternal even carries a comment saying
it must stay consistent with breaker.record, which was in the other
file -- a rule spanning two files with only a comment holding it
together.
Pure move into gate.go: no symbol added or removed.
ab5aa03b9f