Subscribes to entities blueprint lifecycle events and prunes item attachments whose set blueprint was deleted out from under them — the "suspenders" to the delete guard's "belt" (the guard refuses deleting attached sets; this cleans up after deletes that bypassed it, e.g. repo-level operations or a host that never registered the guard). The design doc always claimed this subscriber; the 2026-08-18 panel review noticed it was never actually wired.

Deliberately NOT a boot-time sweep: pruning consults get_set/1, and sweeping while entities is disabled or mid-migration would read every set as missing. prune_orphan_attachments/1 guards on enablement for the same reason. The trade-off: a blueprint deleted while the catalogue app was DOWN is never pruned automatically — the residue is invisible rows (resolve_for_items/2 skips unresolvable sets), recoverable by calling AttributeSets.prune_orphan_attachments/1 with the deleted uuid.