Run jobs an engine enqueues for handlers outside Core without passing the engine on #2632

Part of #2420 (F-CHK) and F-RTE (#2421). Found in the review of #2617.

#2617 makes the job worker refuse a job an engine enqueued when its handler names no engine, so code running as an engine cannot pass the engine on through a job. Today no engine enqueues such a job. Once routing publishes a change set as an engine (ADR-0012), publishing will enqueue jobs for handlers outside Core, such as webhook deliveries. Those would be stored with the engine as their initiator and refused.

Scope

• Decide how a job an engine enqueues for a handler that names no engine runs. For example:
  • as no one, so it cannot act as the engine;
  • or as the person or agent the engine works for, recorded on the job;
  • or through handlers Infrastructure declares to Core.
• Record the choice in ADR-0012.

Acceptance

• A webhook delivery enqueued by a publish that routing makes as an engine runs, and cannot act as the engine.
• A job a check type enqueues while running as the check engine still cannot act as the engine.