[cli] add -o json/yaml to gateway routes and fix parent display format

Refactor the manually-implemented routes command to match the generic
ResourceCommand pattern: add -o json/yaml output, replace duplicated
stdin logic with ReadInputData, scope flag vars locally in
buildRoutesCmd(), and format the PARENTS column as kind://name
(e.g. gateway://my-gw) consistent with domain target display.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dilyevsky

pkg/cmd/gateway/gateway.go

@@ -154,6 +154,6 @@ var gatewayResource = &resource.ResourceCommand[*gatewayv1.Gateway, *gatewayv1.G
 // Cmd returns the gateway command with the routes subcommand attached.
 func Cmd() *cobra.Command {
 	cmd := gatewayResource.Build()
-	cmd.AddCommand(routesCmd)
+	cmd.AddCommand(buildRoutesCmd())
 	return cmd
 }