derive an agent's backend hosts into the egress allow list at the controller

swarm-controller/src/linked_accounts.rs

@@ -621,6 +621,8 @@ pub async fn delete_backend_env(
             e.problem()
         })?;
 
+    crate::backend_egress::follow_env(&state, &agent, None).await?;
+
     tracing::info!(%hive, %agent, "backend env deleted");
     Ok(StatusCode::NO_CONTENT)
 }