Setup prek for CI


```bash
name: Prek checks
on: [push, pull_request]

jobs:
  prek:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v6
      - uses: j178/prek-action@v1
```

https://prek.j178.dev/integrations/#github-actions

This action runs prek on all files. We need to clean all files first...