CLI: auto-create credentials.yaml

Users can manually create a credentials.yaml file inside a project. They can enter key/value pairs under a credential id.

When running a workflow, the CLI will auto-load credential values from this file and feed them into each step, allowing a lightning workflow to execute locally.

But it's annoying for users to hand-build this file.

So when checking out a project, we should auto-populate the credentials.yaml file with whatever credentials are used by the project.

We need to ensure that for every credential id referenced in every workflow for the project, there is an empty value in credentials.yaml.

Do not override any user values. Do add new value if the workflow changed on pull.

Note that we don't need to set any credential values here - just put a stub entry in the file to make life easier for users.