for each agent the hive looks up its record in agent-closures, fetches the closure's narinfo from the cache with the pull token and checks that a signature by the cache's public key covers it. the result is a log line and the gauge hyperhive.agent.closure_pullable, with a reason label when it is 0. nothing is downloaded and deploys still build locally. part 3 of 3: the probe needs the cache facts and the closure read grant from parts 1 and 2.
