docs: document reclaim event and JobExhaustedError usage

Add reclaim event example to the Events section and a "Tracing orphaned
jobs" callout explaining the reclaim → JobExhaustedError chain: how a
mid-run kill leads to a stale-lease reclamation (reclaim event), which on
the next claim may fire a JobExhaustedError on the dead event. Show how to
correlate the two by job id and access lastError for the prior failure.