feat(app): make -json opt-in via App.JSON (v1.3.0)

App now registers -json only when App.JSON is true, so a CLI whose
commands do not use cli.Emit never advertises a flag it cannot honor.
Driven by the first real consumer (evpnc), whose commands print text
directly and are not yet converted to Emit. The example opts in
(JSON: true). Backward-additive: existing App users that want -json set
JSON: true.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app.go

@@ -55,6 +55,10 @@ type App[C any] struct {
 	Root *Node[C]
 	// Greeting, if set, is printed after the version banner in interactive mode.
 	Greeting string
+	// JSON, when true, registers a -json flag that switches Emit to JSON output.
+	// Leave it false for CLIs whose commands do not (yet) use cli.Emit, so they
+	// never advertise a flag they cannot honor.
+	JSON bool
 
 	// DefaultServer is the -server default. If both it and ServerEnv are empty,
 	// no -server flag is registered (local CLI).
@@ -109,7 +113,10 @@ func (a *App[C]) Run(ctx context.Context, argv []string) error {
 		serverFlag = fs.String("server", defaultServer, usage)
 	}
 	color := fs.Bool("color", true, "colorize output (default: on in the shell, off one-shot)")
-	jsonOut := fs.Bool("json", false, "emit JSON instead of text")
+	var jsonOut *bool
+	if a.JSON {
+		jsonOut = fs.Bool("json", false, "emit JSON instead of text")
+	}
 	showVersion := fs.Bool("version", false, "print version and exit")
 	if a.RegisterFlags != nil {
 		a.RegisterFlags(fs)
@@ -125,7 +132,8 @@ func (a *App[C]) Run(ctx context.Context, argv []string) error {
 		fmt.Println(a.versionLine())
 		return nil
 	}
-	if *jsonOut {
+	jsonMode := jsonOut != nil && *jsonOut
+	if jsonMode {
 		SetFormat(FormatJSON)
 	}
 
@@ -145,7 +153,7 @@ func (a *App[C]) Run(ctx context.Context, argv []string) error {
 	if colorExplicit {
 		colorOn = *color
 	}
-	if *jsonOut {
+	if jsonMode {
 		colorOn = false
 	}
 	SetColor(colorOn)