feat: allow replacing buildpack list atomically

Add buildpacks:set --replace so callers can replace an app's complete ordered buildpack list in one command while preserving existing single-buildpack and --index behavior.

Closes #8802

plugins/buildpacks/src/commands/commands.go

@@ -25,6 +25,7 @@ Additional commands:`
     buildpacks:remove <app> <buildpack>, Remove a buildpack set on the app
     buildpacks:report [<app>] [<flag>], Displays a buildpack report for one or more apps
     buildpacks:set [--index 1] <app> <buildpack>, Set new app buildpack at a given position defaulting to the first buildpack if no index is specified
+    buildpacks:set --replace <app> <buildpack> [<buildpack> ...], Replace the entire ordered buildpack list with the specified buildpacks
     buildpacks:set-property [--global|<app>] <key> <value>, Set or clear a buildpacks property for an app`
 )
 

plugins/buildpacks/src/subcommands/subcommands.go

@@ -66,10 +66,11 @@ func main() {
 	case "set":
 		args := flag.NewFlagSet("buildpacks:set", flag.ExitOnError)
 		index := args.Int("index", 0, "--index: the 1-based index of the URL in the list of URLs")
+		replace := args.Bool("replace", false, "--replace: replace the entire ordered buildpack list")
 		args.Parse(os.Args[2:])
 		appName := args.Arg(0)
-		buildpack := args.Arg(1)
-		err = buildpacks.CommandSet(appName, buildpack, *index)
+		_, buildpackList := common.ShiftString(args.Args())
+		err = buildpacks.CommandSet(appName, buildpackList, *index, *replace)
 	case "set-property":
 		args := flag.NewFlagSet("buildpacks:set-property", flag.ExitOnError)
 		global := args.Bool("global", false, "--global: set a global property")