feat(frontend): multiselect/artifact-gate/artifact widgets + linkage + no-limbo

frontend/src/widgets/ArtifactWidget.tsx

@@ -0,0 +1,19 @@
+import type { WidgetProps } from "./types";
+
+/**
+ * ArtifactWidget: view-only renderer. Never calls onRespond.
+ * Invariant no-limbo: this widget intentionally produces no response;
+ * it is used only as a passive display, never as a decision point.
+ */
+export function ArtifactWidget({ descriptor }: WidgetProps) {
+  return (
+    <div className="space-y-2">
+      {descriptor.title && <p className="font-medium">{descriptor.title}</p>}
+      {descriptor.artifact?.content && (
+        <pre className="overflow-auto max-h-64 bg-muted p-3 text-sm rounded border">
+          {descriptor.artifact.content}
+        </pre>
+      )}
+    </div>
+  );
+}