feat: add next-task endpoint for one-task agent loops

[joryirving]

Parent: #392

Add an endpoint that returns exactly one task for an agent.

Behavior:

• worker checks PR follow-up first
• otherwise worker gets one claimable issue in the requested lane
• otherwise worker gets idle
• groomer gets one grooming task when available
• otherwise groomer gets idle

Acceptance:

• no LLM required to check for work
• one task per response
• compatible with existing queue logic
• idle response lets any harness stop cleanly