“Jones” is an American slang word meaning to be addicted to something, so “jonesing” for something means to crave something very strongly, and generally very vocally.

“Breve” is a coffee drink that is commonly made with half-and-half, which is a product that is equal parts cream and milk. I assume that people have taken to using the term to refer to half-and-half itself, but I’ve not personally heard that.

So the sentence is saying that their cat was addicted to half-and-half and would act like a junkie doing anything to get their next fix.

In case you missed the joke, this is a reference to how John Hinckley Jr shot Reagan because he thought it would impress Jodie Foster. It’s posted today because of the assassination attempt on Trump yesterday.