Checks out a stack by stack number, pull request number, pull request URL, or
locally tracked branch. A target is required so the command never opens the
interactive `gh stack` picker.
Usage:
```bash
Without `--yes`, Quinjet prints a preview and does not change the checkout.
```bash
quinjet stack checkout 42 --yes
```
See [`quinjet stack`](./README.md) for lifecycle safety and prerequisites.