The top-level behavior is to run through the entire contents of _episodes in lexical order, I think, which is a reasonable default, but making it explicit probably improves legibility. The downside is that it's yet another place where the file-names are hard-coded.

On balance, I favor this, and have now done it.

Yeah... I prefer having the episode order defined explicitly at the top, then we can document changes. When the site builds on GitHub, it only uses what's in _config.yml, but local copies -- and those on GitLab, which provides a config file for the build system -- can use HPC_JEKYLL_CONFIG to override the top-level settings. If a site's snippet config defines a different order (like CTCMS dropping the modules lesson), those local changes are kept. Otherwise, default; or, as is currently the case, every snippet lib also explicitly defines lesson order.