You will have to use a static list of your folder contents if you want to avoid dealing with access tokens and Github’s API. Edit: Not true, see If Observable authenticates via GitHub, can I easily write to it?).
The metadata at the top of markdown files is often referred to as “frontmatter”. Reading it currently requires fetching each individual file. One could probably stream each file up to the second -- delimiter, but a better solution might be to generate an index file that contains the frontmatter and file name of each file, and to put it in the same directory. Then you’d only have to fetch and read that index file.