List an operating-memory item's removable shares

Returns the removable share grants on one Operating Memory item -- the people, teams, or whole workspace a human explicitly shared it with through Operating Memory and can take back with the unshare operation. Node-level share grants only: an inherited source audience and an ingest-resolved audience (meeting participants, Slack members, the tenant-wide grant on external sources) are not removable here and are not returned. Keyed by node_uuid; returns 404 when the node is not visible to you, and does not distinguish absent from invisible.