fix: move skills into .claude-plugin to avoid path traversal

Author: Yvictor

.claude-plugin/marketplace.json

@@ -7,7 +7,7 @@
   "plugins": [
     {
       "name": "polars-backtest",
-      "source": "./..",
+      "source": "./",
       "description": "Polars backtest extension development helper - backtesting with polars expressions"
     }
   ]