chore(copilot): drop dead pnpm config from package.json

The `pnpm.onlyBuiltDependencies` block is only honored by pnpm and the
project uses npm (per `package-lock.json` and the `npm ci` deploy path).
Removing dead config; no functional change.

Author: bendersej

copilot/package.json

@@ -65,11 +65,5 @@
     "typescript": "^5.7.2",
     "vite": "^8.0.0",
     "vitest": "^3.0.5"
-  },
-  "pnpm": {
-    "onlyBuiltDependencies": [
-      "esbuild",
-      "lightningcss"
-    ]
   }
 }