Add core plugin to marketplace.json

Author: varin-nair-factory

.factory-plugin/marketplace.json

@@ -17,6 +17,12 @@
       "description": "Skills for continuous learning and improvement: session navigation, human writing, skill creation, visual design, and browser automation",
       "source": "./plugins/droid-evolved",
       "category": "productivity"
+    },
+    {
+      "name": "core",
+      "description": "Core Skills for essential functionalities and integrations",
+      "source": "./plugins/core",
+      "category": "core"
     }
   ]
 }