Cutting our CI time in half thanks to custom AWS runners

We moved our Nx monorepo from CircleCI to GitHub Actions on self-hosted AWS runners, tightened caching, and brought the full CI run from ~33 minutes down to ~15 minutes while owning cache and runner behavior end to end. At Wecasa, our CI used to run on CircleCI. We wanted to consolidate on GitHub Actions and run jobs on self-hosted runners in AWS, with specs we control and costs we can optimize. Our repo is a large Nx monorepo: each package is an app or library (SPAs, React Native / Expo mobile