mirror Discord replies as replies on X and bluesky

Replying to a message on the watched channel now posts the reply as a
reply to the posts that message produced, instead of as a standalone
post.

Discord tells us the parent through MessageReference, and the store maps
it back to the Ref each output returned; that Ref rides along on the new
Post.ReplyTo. X takes it as in_reply_to_tweet_id. Bluesky needs the
thread root as well as the parent, so a Ref now carries the root it was
posted under and a post that is not itself a reply acts as its own root.

Replying to something we never distributed, an error notice from the bot
for instance, still posts normally.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

output/twitter.go

@@ -63,6 +63,12 @@ func (to *twitteroutput) Write(post Post) (Ref, error) {
 			MediaIDs: mediaIDs,
 		}
 	}
+	if post.ReplyTo != nil && post.ReplyTo.ID != "" {
+		p.Reply = &types.CreateInputReply{
+			InReplyToTweetID: post.ReplyTo.ID,
+		}
+	}
+
 	out, err := managetweet.Create(context.Background(), to.client, p)
 	if err != nil {
 		return Ref{}, err