feat(format): spell Discord's message markup once

[mikield]

Mentions, timestamps and custom emoji get assembled by hand wherever a message is built, and one assembled slightly wrong renders as literal text rather than failing — the bot shows <@123> to its users and nothing in the logs says why. Format holds the shapes so they are written correctly in one place.

Format::user($userId);                                  // <@254766810296090626>
Format::channel($channelId);                            // <#979052732218503179>
Format::command('voice room limit', $commandId);        // </voice room limit:12345>
Format::emoji('apex', $emojiId, animated: true);        // <a:apex:12345>
Format::timestamp($when, TimestampStyle::Relative);     // <t:1618928400:R>
Format::codeBlock($sql, 'sql');

Two things worth pointing out:

• Timestamp styles are an enum, not the raw letter. Nothing about 'R' says "2 months ago" at a call site, and the letters are case-sensitive in a way that is easy to get wrong (d and D are different formats). The default matches Discord's own default when no style is given.
• timestamp() takes int|DateTimeInterface. Seconds because that is what the API takes, and a native date because that is what most callers have. It deliberately knows nothing about any particular datetime library.

This came out of a bot that had grown its own copy of exactly these five helpers.

🤖 Generated with Claude Code

[github-actions]

🎉 This PR is included in version 1.2.0 🎉

The release is available on GitHub release

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