test(cli): add --plain override tests and TTY limitation comment

Add tests verifying --plain overrides GOG_FORCE_OUTPUT and
--non-interactive auto-JSON. Add comment explaining why TTY-positive
path cannot be tested (captureStdout uses os.Pipe, always non-TTY).

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

Author: salmonumbrella

internal/cmd/root_autojson_test.go

@@ -5,6 +5,11 @@ import (
 	"testing"
 )
 
+// NOTE: captureStdout replaces os.Stdout with an os.Pipe, which is always
+// non-TTY. This means we can test the "piped stdout → auto-JSON" path but
+// cannot test the "real TTY → no auto-JSON" path in unit tests. The TTY
+// guard (term.IsTerminal) is effectively always false here.
+
 func TestAutoJSON_Version_DefaultsToJSONWhenEnabled(t *testing.T) {
 	t.Setenv("GOG_AUTO_JSON", "1")
 
@@ -59,6 +64,36 @@ func TestAutoJSON_Version_DefaultsToJSONWhenForceOutputEnabled(t *testing.T) {
 	}
 }
 
+func TestAutoJSON_Version_PlainOverridesForceOutput(t *testing.T) {
+	t.Setenv("GOG_FORCE_OUTPUT", "1")
+
+	out := captureStdout(t, func() {
+		_ = captureStderr(t, func() {
+			if err := Execute([]string{"--plain", "version"}); err != nil {
+				t.Fatalf("Execute: %v", err)
+			}
+		})
+	})
+
+	if strings.HasPrefix(strings.TrimSpace(out), "{") || strings.Contains(out, "\"version\"") {
+		t.Fatalf("expected text output (not json), got: %q", out)
+	}
+}
+
+func TestAutoJSON_Version_PlainOverridesNonInteractive(t *testing.T) {
+	out := captureStdout(t, func() {
+		_ = captureStderr(t, func() {
+			if err := Execute([]string{"--non-interactive", "--plain", "version"}); err != nil {
+				t.Fatalf("Execute: %v", err)
+			}
+		})
+	})
+
+	if strings.HasPrefix(strings.TrimSpace(out), "{") || strings.Contains(out, "\"version\"") {
+		t.Fatalf("expected text output (not json), got: %q", out)
+	}
+}
+
 func TestAutoJSON_Version_DefaultsToJSONWhenNoInputAndPiped(t *testing.T) {
 	out := captureStdout(t, func() {
 		_ = captureStderr(t, func() {