API layer (e.g., GraphQL) over Markdown files

Been trying to figure this out for two days and must be thinking about it incorrectly. I'd like to support the following workflow:

- write in markdown with YAML frontmatter (or similar)

- query files and frontmatter from different apps & sites through an API (ideally GraphQL)

It seems this word is divided into two segments:

1. Git-based CMS like Forestry, Grav, Netlify which allows you to source from git flat files, modify and deploy in their ecosystem as a static site

2. Headless CMS's like Contentful, Strapi, etc that expose an API layer atop their CMS database (or in the case of Hasura, atop Postgres, MySQL, etc)

There are many examples of sourcing content from a headless CMS to a static site or git-based CMS, but not vice-versa. For example, you can have a Gatsby + Contentful site deployed on Netlify, but in this case, you're sourcing front Contentful to Gatsby (which can also source from flat files / git), but you can't then fetch them from a third party app.

I already maintain my content in git as markdown, and my aim is to fetch that content (paginate, filter by frontmatter, etc) in a Nuxt app, in Webflow, on a blog (e.g., static site). I could potentially fetch the various repository's files through Git's API, but then I'd have a ton of client-side logic in each app or place I'd like to use that content.

Effectively I'm looking for how to set up a thin API layer over flat files (hosted on Git or otherwise). Like a Hasura with markdown as the source. Like a Headless CMS that can source and/or sync with flat files. (Or to set this up myself.)