Update marketplace.json for playground rename

.claude-plugin/marketplace.json

@@ -362,15 +362,15 @@
       "homepage": "https://github.com/anthropics/claude-plugins-public/tree/main/plugins/frontend-design"
     },
     {
-      "name": "artifact",
-      "description": "Creates interactive HTML artifacts — self-contained single-file explorers with visual controls, live preview, and prompt output with copy button. Includes templates for design playgrounds, data explorers, concept maps, and writing tools.",
+      "name": "playground",
+      "description": "Creates interactive HTML playgrounds — self-contained single-file explorers with visual controls, live preview, and prompt output with copy button. Includes templates for design playgrounds, data explorers, concept maps, and document critique.",
       "author": {
         "name": "Anthropic",
         "email": "support@anthropic.com"
       },
-      "source": "./plugins/artifact",
+      "source": "./plugins/playground",
       "category": "development",
-      "homepage": "https://github.com/anthropics/claude-plugins-official/tree/main/plugins/artifact"
+      "homepage": "https://github.com/anthropics/claude-plugins-official/tree/main/plugins/playground"
     },
     {
       "name": "ralph-loop",