chore: migrate tooling from Bun to pnpm - #3

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11d5960a326750d5838078e36cf38b85af677262, pnpm/action-setup@f40ffcd9367d9f12939873eb1018b921a783ffaa. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/