fix(core): refactor the old sanitizer to use discord.py's built-in

[MikkelHebel]

Summary

Replaces the old markdown/mention sanitizer with discord.py's own built-in.

• markdown_clean now uses discord.utils.escape_markdown
• discord_clean now uses discord.utils.escape_markdown + discord.utils.escape_mentions, plus a small added regex to also break channel references (<#id>), since discord.py's mention escaping only covers @everyone/@here/user/role mentions and not channel links
• Also configures the client with allowed_mentions=discord.AllowedMentions.none() as a second line of defense against pings.

Note: this escapes less aggressively than the old version, it only touches constructs that actually render as Markdown or a mention/reference, instead of blanket-escaping characters like {, (, ., ! that never meant anything in context. Same real coverage (formatting breakout, all four mention/reference types), just no more escaping for the sake of it.

This closes #7

[The0mikkel]

I'm setting this on hold, until #16 have been merged, as this PR needs to be updated aftwards.