This diff (and the relevant Phabricator diff) does three things:
- Migrates BuildStepImplementation and SleepBuildStepImplementation into Arcanist.
- Introduces arc agent; the intention is that this will be run on build agents to listen for and run builds.
- Introduces agent.nextbuild Conduit call, which currently just states there's no builds to pick up.
I am not quite sure how we're going to reconcile the worker (which picks up builds immediately), and agents which have to poll (because Phabricator has no direct way of sending a message to the agent immediately).