Delete a quarantine item without releasing

Marks the item deleted. The underlying content hash is retained for audit. No refund of the original (blocked) prompt to the calling agent. Authorization: the caller must be authenticated, resolve to an organization and hold the owner, admin or member role in it (viewers and auditors are read-only and get 403), and quarantine_id is matched scoped to that org's own agents, so a record owned by another tenant returns 404 and no other tenant's item can be deleted.