Add timed prompt option

[JeremyCarlsten]

We already have schedules so this should be easy. But periodically I hit the session limit on Claude. It's be really nice if I could create a future prompt.

I kick off a job and halfway through it hits session timeout. (Really only a Claude problem). But my session doesn't reset until 12am. Maybe I can schedule my continue prompt for 12:01 and have some work done when I wake up in the am.

[Herbrant]

Same need, one thread: #4494 collects this alongside the recurring case from #3748 and #4122, framed as a workflow. Your exact case — kick off a job, hit the session limit, queue the continue prompt for 00:01 — is in there as the one-shot shape.