Deno CI on GitHub Actions

Deno re-downloads and re-type-checks remote modules on every run.

Why it happens

DENO_DIR holds the module and type-check cache and is not persisted.

What to change

1. Cache DENO_DIR keyed on deno.lock
2. Run deno cache before tests so the download is a separate, visible step
3. Use --no-check in test runs where types are already verified elsewhere

Deno on runnerhut

jobs:
  test:
    runs-on: runnerhut-4vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: denoland/setup-deno@v2
      - run: deno cache main.ts
      - run: deno test -A --parallel

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.