Periodically deletes terminal (done/failed) instances older than the retention window (spec §8) — the engine's built-in pruner.

Each sweep removes at most :gc_batch rows (Queries.gc/3); if it fills the batch, more remain, so it re-sweeps immediately to drain a backlog rather than waiting a full interval. A terminal child whose parent is still mid-join is spared — see Queries.gc/3.

Disable by passing gc_interval: nil to the engine; the GC process is then not started and terminal rows accumulate until pruned externally.