Drop packageManager field from package.json

The pinned yarn@4.0.2 didn't match the Yarn 1 classic install/lockfile
actually used. Remove it to avoid Corepack switching package managers;
this no-build project just uses `node bs.js`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

package.json

@@ -9,6 +9,5 @@
   "dependencies": {
     "browser-sync": "^2.29.3",
     "bs-rewrite-rules": "^2.1.2"
-  },
+  }
-  "packageManager": "yarn@4.0.2"
 }