ci: pin packageManager so pnpm/action-setup picks a version

pnpm/action-setup@v4 fails with "No pnpm version is specified" unless one of the action's `version` input or package.json `packageManager` is set. Using `packageManager` keeps CI and local installs aligned through one source of truth.

Amp-Thread-ID: https://ampcode.com/threads/T-019df512-66f8-71aa-8696-9ad96563f39a

Author: jxom

package.json

@@ -2,6 +2,7 @@
   "name": "wevm.dev",
   "private": true,
   "type": "module",
+  "packageManager": "pnpm@10.33.0",
   "sideEffects": false,
   "scripts": {
     "dev": "vp dev",