Ideally, GitHub should have a pragma to include an inline TOC ...

Oh. That reminds me... Asciidoc (rather than Markdown) has that capability.

For example, when the "WebAssembly" wiki page on the Go wiki became pretty long, I converted it from Markdown to Asciidoc specifically so it could autogenerate the ToC.

The first few lines of that file (in raw text mode) are now:

WebAssembly
===========
:toc:
:toc-title:
:toclevels: 2
:icons:

Which generates the Table of Contents seen here: https://github.com/golang/go/wiki/WebAssembly

That being said, I'm not really a fan of Asciidoc otherwise. Markdown seems more natural (to me), though that's probably just a familiarity thing.