High-priority execution tier for time-sensitive agents

When an agent is triggered, it can take up to a couple of minutes to start while the platform gets everything ready. For most automations that's fine, but for time-sensitive work — dispatching, urgent customer-facing tasks — those minutes matter.

The idea: let us mark certain agents as high priority (possibly at a higher credit cost) so their runs start faster and jump ahead of routine background work.

Requested by a customer using Autotask.