Is there a rule how to tag a bug-fix in a change-log, where the bug was reported by someone on GitHub or CPAN?
MetaCPAN automatically converts text that matches RT#12345 (where 12345 is a ticket number) into a link to rt.cpan.org. Without the "RT", I believe a bit of text like #12345 is linked to your preferred bug tracker (as given in META.json), which could be, for example, your GitHub issues page. I'm not 100% sure on that though, as I prefer all my bugs to be reported to rt.cpan.org.
Either way, I tend to include an explicit URL too for the benefit of those people who are not reading the changelog on MetaCPAN. I'll also note who reported the bug.
Sometimes somebody will tell me about a bug report via IRC or e-mail, so it won't have an issue number. In which case, I note who reported it, but provide no link.
Here's an example of one of my changelogs. Bug reporters also end up in my CREDITS files too!