Markdown in GitHub entity titles are not sanitised

[00-kat]

Some screenshots of what this causes:

A search for 00-kat/test in the Ghostty bot testing Discord server reveals quite a few too, some of which are shown below. Not all of them are related to Markdown sanitisation but rather general sanitisation of Discord features embedded with special syntax in text messages.


This one required <@user id>, very unlikely apart from intentional abuse, so should likely not be handled. Included for completeness.

Embeds could be very large, this one was luckily small.

Also contrived.

Not as contrived since GitHub also has :emojiname: syntax, and someone who doesn't know GitHub doesn't parse titles may try using them. Though one could argue this is preferable.

Also contrived, but people have a reason to abuse this as it may aid in causing harm.

[00-kat]

This is actually easier than I had thought; I had collected a list of characters to escape, but then found discord.utils.escape_markdown and discord.utils.escape_mentions.