Looking at the documentation here, the title object does not allow access to the page's short description. Currently (on en.wiki), in order to get the short description of a page that uses a nested short description template inside of another template (such an Infobox), title.getContent() is used and then the return article text is searched in order to find the short description text. Directly accessing the value from title.getDescription() would allow for a much cleaner code. Apologizes if there is already a task like this, but I couldn't find one.
Short description handling currently lives in Wikibase, so mw.wikibase makes sense nevertheless.
This should be reasonably easy, there's a service to return the description, it just needs some scaffolding to make it available to Lua.
Just to repeat myself, again, mw.wikibase is completely irrelevant to this, as is Wikidata. I'm not trying to access the description at Wikidata, for that I can use mw.wikibase.getDescription, which is already available. I'm trying to access the local description which can be seen either on the article itself, or in the "Page information" page. That can give a completely different description.