fix: add packageManager field to package.json

pnpm/action-setup@v5 requires either a version key in the action config
or a packageManager field in package.json. Add the field to unblock the
release workflow.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

package.json

@@ -17,6 +17,7 @@
   "homepage": "https://github.com/privilegedescalation/headlamp-sealed-secrets-plugin#readme",
   "author": "privilegedescalation",
   "license": "Apache-2.0",
+  "packageManager": "pnpm@10.32.1",
   "scripts": {
     "start": "headlamp-plugin start",
     "build": "headlamp-plugin build",