CI/CD setup can be a list of layers

setup: in runtime.yml now also accepts a list of up to 16 commands, run in order. A push to the production branch keeps each finished layer as a snapshot, keyed to that layer, the layers before it, the machine size and the cache files. A later setup starts from the deepest layer that is ready and runs only the layers after it, so a change to the last layer does not run the earlier ones again, and a setup that is stopped or fails partway keeps the layers it finished. Pull request checks start from these snapshots but never write them. A setup: of one command works as before.