CI/CD checks every push of the branches you list

Add pushes: [{ branch: next, steps: [...] }] to the production branch's .runtime/checks.yml to check each push of a branch that has no pull request of its own, such as a staging branch. Each such check runs only the named steps, appears on GitHub as runtime @ <branch> rather than runtime, and once started it runs to its own verdict instead of being replaced by a newer push.