Let root-free take its machines as a list, as the router names them

The router's contract names the machines as a JSON array. A verb's argument declared a list now takes
an array of names (or one text separated by commas), and refuses anything else in it.

cmd/mesh-controller/seatverbs.go

@@ -114,6 +114,14 @@ func declaredArguments(v catalogue.Verb) (names []string, switches map[string]bo
 	return names, switches
 }
 
+// isList says a verb's argument is declared a list of text (catalogue's listed): given as a JSON array, it is
+// read as its items joined by commas, as the same argument given as one text would be.
+func isList(v catalogue.Verb, name string) bool {
+	props, _ := v.Input["properties"].(map[string]any)
+	p, _ := props[name].(map[string]any)
+	return p != nil && p["type"] == "array"
+}
+
 // readArguments refuses what the verb does not take, before anything is composed.
 func readArguments(verb string, args map[string]any) (*verbArguments, error) {
 	v, known := controllerVerb(verb)
@@ -150,6 +158,19 @@ func readArguments(verb string, args map[string]any) (*verbArguments, error) {
 				return nil, fmt.Errorf("%s: %q is text, not true or false", verb, k)
 			}
 			value = fmt.Sprint(x)
+		case []any:
+			if !isList(v, k) {
+				return nil, fmt.Errorf("%s: %q is text, and was given a list", verb, k)
+			}
+			items := make([]string, 0, len(x))
+			for _, item := range x {
+				text, ok := item.(string)
+				if !ok || strings.TrimSpace(text) == "" || strings.Contains(text, ",") {
+					return nil, fmt.Errorf("%s: %q is a list of names, and holds %v", verb, k, item)
+				}
+				items = append(items, strings.TrimSpace(text))
+			}
+			value = strings.Join(items, ",")
 		default:
 			return nil, fmt.Errorf("%s: %q is text, and was given %T", verb, k, x)
 		}