check for existence of custom job ymls when generating ci

Right now if you typo your ci yml name (or use ".yaml" instead of ".yml") you get latebound errors in github CI. We should check for the existence of the scripts when generating and either:

* Error out if they're missing
* Fill in the greeter example to get the user started

Start at the CI generation path that resolves custom job YAML names and inspect how the greeter example is populated. Reproduce the failure with a missing or .yaml-named job, then verify that generation either reports the missing script immediately or provides the greeter example as described.

Written by the indexing model from the issue text.