The registered cells, with reachability: {"cells": [{id, label, address, host, credential_ref, model_id, backend, configuration_digest, source, reachable, error}], "active_cell": "…"}.

POST /dense-execution/cells only accepts a cell that matches a commissioned server-side binding (otherwise cell_not_commissioned or cell_binding_pinned). DELETE /dense-execution/cells/{id} always refuses with cell_commissioning_required, or dense_cell_active for the selected cell: commissioned cells are removed from the catalogue file, not from the UI.