I think it would look exactly the same as it does on a PR. Just a comment on the issue with the same text (This issue has been mentioned on Discourse Meta. There might be relevant details there.\n\n\<link>). Maybe you are thinking of a reason that you may think that text would need to change as it’s extremely likely I am not thinking all reasons through.
I’d say just one setting titled “github linkback on issues enabled” with a checkbox.
I am open to a PR that adds support for something here, getting security right is tricky, we have done something in discourse code review but you need a token and you don’t want it to be your token, instead one of a throwaway github account that has no access to repos you care about.