fix: upgrade pnpm to 9.15.9 to fix ERR_INVALID_THIS with Node 24

pnpm 7.x has a fetch bug that causes ERR_INVALID_THIS errors when
downloading packages using Node.js 22+. Upgrade to pnpm 9 fixes it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: unional

package.json

@@ -13,5 +13,5 @@
     "@changesets/cli": "^2.25.2",
     "turbo": "^1.6.3"
   },
-  "packageManager": "pnpm@7.27.1"
+  "packageManager": "pnpm@9.15.9"
 }