cluster: derive default machine name from its hostname when adding to cluster

cmd/uncloud/machine/add.go

@@ -71,7 +71,8 @@ Connection methods:
 			return add(cmd.Context(), uncli, remoteMachine, opts)
 		},
 	}
-	cmd.Flags().StringVarP(&opts.name, "name", "n", "", "Assign a name to the machine.")
+	cmd.Flags().StringVarP(&opts.name, "name", "n", "",
+		"Assign a name to the machine. (default is the machine's hostname)")
 	cmd.Flags().BoolVar(
 		&opts.noCaddy, "no-caddy", false,
 		"Don't deploy Caddy reverse proxy service to the machine.",

cmd/uncloud/machine/init.go

@@ -101,7 +101,7 @@ Connection methods:
 		"API endpoint for the Uncloud DNS service.")
 	cmd.Flags().StringVarP(
 		&opts.name, "name", "n", "",
-		"Assign a name to the machine.",
+		"Assign a name to the machine. (default is the machine's hostname)",
 	)
 	cmd.Flags().StringVar(
 		&opts.network, "network", cluster.DefaultNetwork.String(),