Airflow CI on GitHub Actions

DAG import checks reinstall the full Airflow dependency set each run.

Why it happens

Airflow's constraint-pinned dependency tree is large and is installed from scratch every time.

What to change

1. Cache the virtualenv keyed on the constraints file
2. Run DAG validation without a scheduler using the standalone DagBag check
3. Use a prebuilt Airflow image rather than pip installing in the job

Airflow on runnerhut

jobs:
  validate:
    runs-on: runnerhut-4vcpu-ubuntu-2404
    container: apache/airflow:2.10.0
    steps:
      - uses: actions/checkout@v4
      - run: python -c "from airflow.models import DagBag; assert not DagBag().import_errors"

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.