Pants CI on GitHub Actions

Pants re-runs every process because the local cache is empty.

Why it happens

~/.cache/pants holds the named caches and local process cache and is not persisted.

What to change

1. Cache ~/.cache/pants/named_caches and the lmdb store
2. Configure a remote cache so results are shared across jobs
3. Use pants --changed-since to run only affected targets

Pants on runnerhut

jobs:
  test:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
        with: { fetch-depth: 0 }
      - run: pants --changed-since=origin/main test

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.