No account needed. It just works off local markdown files.

No account needed. They don't have access to your stuff—it works on local files—but appear to be working on a cloud option. They claim to support Commonmark and Github Flavored Markdown. [[filename]] is just a shortcut for [filename](filename), but in any case "Export to standard Markdown links" is on their short-term roadmap.

You might also check out Zim.

I believe in yesterdays thread there was mention of the app storing all files locally in a portable Markdown format

Have you heard anything about Joplin? I believe it behaves similarly.