Go CI on GitHub Actions

Go builds are fast locally and inexplicably slow in CI.

Why it happens

The Go build cache lives in ~/.cache/go-build and is empty on every fresh runner, so every package compiles from scratch.

What to change

1. Cache ~/.cache/go-build and ~/go/pkg/mod keyed on go.sum
2. Run go build ./... before go test so the build cache is populated once
3. Use -count=1 deliberately — it disables the test result cache, so only use it when you mean it
4. Shard large test suites with -run and a matrix rather than buying a bigger machine

Go on runnerhut

jobs:
  test:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-go@v5
        with: { go-version: '1.24', cache: true }
      - run: go build ./...
      - run: go test -race ./...

The only runnerhut-specific line is `runs-on: runnerhut-8vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.