Bazel CI on GitHub Actions

Bazel's whole value is caching, and CI throws the cache away every run.

Why it happens

The local disk cache and repository cache are not persisted, so every action re-executes despite Bazel knowing better.

What to change

1. Point --disk_cache at a persisted directory and cache it between runs
2. Add a remote cache so every developer and every CI job share results
3. Cache the repository cache so external dependencies are not re-fetched
4. Use --jobs matched to the runner core count

Bazel on runnerhut

jobs:
  build:
    runs-on: runnerhut-16vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - run: |
          bazel build //... \
            --disk_cache=~/.cache/bazel \
            --repository_cache=~/.cache/bazel-repo \
            --jobs=16

The only runnerhut-specific line is `runs-on: runnerhut-16vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.