fix skill-creator install: use anthropics/skills marketplace

skill-creator is bundled in the "example-skills" plugin from
anthropics/skills repo, not in anthropics/claude-code marketplace.
Adds anthropic-agent-skills marketplace and installs example-skills.

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

settings.json

@@ -233,6 +233,15 @@
         "source": "github",
         "repo": "nextlevelbuilder/ui-ux-pro-max-skill"
       }
+    },
+    "anthropic-agent-skills": {
+      "source": {
+        "source": "github",
+        "repo": "anthropics/skills"
+      }
     }
+  },
+  "enabledPlugins": {
+    "example-skills@anthropic-agent-skills": true
   }
 }