Elixir CI on GitHub Actions

mix deps.get and compilation both start from nothing each run.

Why it happens

deps/ and _build/ are not cached, so every dependency recompiles even though none changed.

What to change

1. Cache deps/ and _build/ keyed on mix.lock and the OTP/Elixir versions
2. Compile with MIX_ENV=test once and reuse for both credo and tests
3. Run --warnings-as-errors in a separate fast job so it fails early
4. Use ExUnit's built-in async tests — most suites leave them off by accident

Elixir on runnerhut

jobs:
  test:
    runs-on: runnerhut-4vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: erlef/setup-beam@v1
        with: { otp-version: '27', elixir-version: '1.18' }
      - run: mix deps.get
      - run: mix test

The only runnerhut-specific line is `runs-on: runnerhut-4vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.