feat(channel): modify a thread, tags and all

[mikield]

A forum post's tags can be read off Channel::$applied_tags but never written: no builder carries the field, so there is no way to move a post from "voting" to "accepted" through the library. Every forum workflow turns on exactly that.

$discord->rest->channel->modify($threadId, ThreadChannelBuilder::new()
    ->setAppliedTags([$acceptedTagId])
    ->setLocked(true));

Why its own builder rather than a trait on the existing ones. A thread is not an ordinary channel: it has no topic, no permission overwrites and no parent to move it between, and it has archiving, locking, invitability and tags, which no other channel has. Handing a thread to GuildTextChannelBuilder sends type and whatever else was set — fields Discord rejects for a thread — and offers none of the ones that actually apply.

applied_tags replaces the whole set, because that is what Discord does with the field: a tag left out of the list comes off the post. It is reindexed on the way out, since an array with gaps serialises to a JSON object rather than an array and the call comes back refused — which is the kind of thing you only find in production, after filtering a tag list.

Found while porting a bot whose petitions live in a forum and move between four tags over their life.

🤖 Generated with Claude Code

[github-actions]

🎉 This PR is included in version 1.3.0 🎉

The release is available on GitHub release

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