feat: expose cheap idle checks for scheduled harnesses

[joryirving]

Parent: #392

Make idle cheap and explicit so scheduled harnesses can stop before starting a model.

Expected behavior:

• next-task returns idle when no work exists
• idle response includes a short reason
• idle check is read-only
• idle check does not require model context

Acceptance:

• worker schedule can exit early
• groomer schedule can exit early
• tests cover empty queue and empty follow-up state
• docs show the idle-first pattern