Are you asking whether Github is offering their agent software for others to use? Or just whether, in general, there are agents which can make use of Let's Encrypt?

The answer to the latter is yes, of course, a wide variety of suitable software exists, including shell scripts like https://acme.sh/ the "official" Certbot Python code and software like Caddy which implements this feature right inside a web server.

Mailinabox (https://mailinabox.email/guide.html#admin) uses it to get SSL certificates too; you can see it in action here https://github.com/mail-in-a-box/mailinabox/blob/master/mana...