Regarding:
"So I commit an .md or .htm file ..."
Retejo supports .md and .htm, but also supports all the other file-extension listed here:
https://codeberg.org/reiver/go-ext2media/src/branch/master/init.go
Regarding:
"... and that results in an AP post"
That results in Retejo returning content for whatever path you created that file at.
And, that path becomes a first-class object on the Fediverse and Social-Web.
But, at least currently, Retejo does NOT post an activity about it to anyone else's ActivityPub inbox.