Add enforce.Guard port with X11 and no-op adapters

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

internal/enforce/x11_integration_test.go

@@ -0,0 +1,23 @@
+//go:build linux
+
+package enforce
+
+import (
+	"context"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestX11GuardMinimizeActiveDoesNotPanic(t *testing.T) {
+	if os.Getenv("DISPLAY") == "" {
+		t.Skip("no DISPLAY; skipping live X11 minimize smoke test")
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	// Either it minimizes the active window or returns an error (e.g. no active
+	// window); we only assert it returns without panicking.
+	if err := NewGuard().MinimizeActive(ctx); err != nil {
+		t.Logf("MinimizeActive returned (acceptable): %v", err)
+	}
+}