Remove bundled Artemis automation skill

.agents/plugins/marketplace.json

@@ -160,18 +160,6 @@
       },
       "category": "Productivity"
     },
-    {
-      "name": "artemis-automation",
-      "source": {
-        "source": "local",
-        "path": "./plugins/codex/plugins/artemis-automation"
-      },
-      "policy": {
-        "installation": "AVAILABLE",
-        "authentication": "ON_INSTALL"
-      },
-      "category": "Developer Tools"
-    },
     {
       "name": "artemis-case-prep",
       "source": {