Update packageManager to pnpm 9.15.9

Fixes CI error where GitHub Action uses pnpm 9 but package.json specified pnpm@8.15.0

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dokterbob

package.json

@@ -40,7 +40,7 @@
     "node": ">=18",
     "pnpm": ">=8"
   },
-  "packageManager": "pnpm@8.15.0",
+  "packageManager": "pnpm@9.15.9",
   "pnpm": {
     "onlyBuiltDependencies": [
       "esbuild"