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

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

internal/enforce/guard_other.go

@@ -0,0 +1,12 @@
+//go:build !linux
+
+package enforce
+
+import "context"
+
+// NewGuard returns a no-op guard on platforms without the X11 adapter.
+func NewGuard() Guard { return noopGuard{} }
+
+type noopGuard struct{}
+
+func (noopGuard) MinimizeActive(context.Context) error { return nil }