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>

discord/discordread.go

@@ -20,6 +20,9 @@ type Event struct {
 	MessageID   string
 	Content     string
 	Attachments []*discordgo.MessageAttachment
+	// ReplyToID is the ID of the message this one replies to, empty when the
+	// message is not a reply.
+	ReplyToID string
 }
 
 type Client struct {
@@ -49,12 +52,16 @@ func (d *Client) read(eventchannel chan<- Event) {
 		if m.ChannelID != d.ChannelID || m.Author.ID == s.State.User.ID {
 			return
 		}
-		eventchannel <- Event{
+		event := Event{
 			Kind:        MessageCreated,
 			MessageID:   m.ID,
 			Content:     m.Content,
 			Attachments: m.Attachments,
 		}
+		if m.MessageReference != nil {
+			event.ReplyToID = m.MessageReference.MessageID
+		}
+		eventchannel <- event
 	})
 
 	// Deleted messages arrive without an author, so the channel is the only