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

Each sweep removes at most 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.

Configured per node via the engine's :gc option ([interval:, retention:, batch:], or false to not run GC on this node — at least one node in the cluster must, or terminal rows and stale limiter counters accumulate forever).