AWS CDK CI on GitHub Actions

cdk synth reinstalls dependencies and re-synthesises every stack.

Why it happens

node_modules and the CDK context cache are cold on each run.

What to change

1. Cache node_modules and cdk.out keyed on the lockfile
2. Synthesise once and reuse the assembly for diff and deploy
3. Use --exclusively to deploy a single stack rather than the whole app

AWS CDK on runnerhut

jobs:
  deploy:
    runs-on: runnerhut-4vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - run: npm ci
      - run: npx cdk synth
      - run: npx cdk deploy --require-approval never --app cdk.out

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.