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>

main.go

@@ -114,7 +114,8 @@ func (dist *distributor) reportf(format string, args ...any) {
 	dist.d.Write(errstr)
 }
 
-// created posts a new Discord message to every output.
+// created posts a new Discord message to every output, as a reply when the
+// Discord message itself was a reply to something we already distributed.
 func (dist *distributor) created(event discord.Event) {
 	if length := utf8.RuneCountInString(event.Content); length > maxTweetLength {
 		dist.reportf("Error: message is %d characters, exceeding the %d character limit; not posted", length, maxTweetLength)
@@ -131,12 +132,23 @@ func (dist *distributor) created(event discord.Event) {
 		return
 	}
 
+	// A reply to a message we never distributed becomes a top level post.
+	var parents store.Refs
+	if event.ReplyToID != "" {
+		parents, _ = dist.store.Get(event.ReplyToID)
+	}
+
 	refs := store.Refs{}
 	for _, out := range dist.outputs {
-		ref, err := out.Write(output.Post{
+		post := output.Post{
 			Text:   event.Content,
 			Images: images,
-		})
+		}
+		if parent, ok := parents[out.GetName()]; ok && !parent.IsZero() {
+			post.ReplyTo = &parent
+		}
+
+		ref, err := out.Write(post)
 		if err != nil {
 			dist.reportf("%s Error: %s", out.GetName(), err)
 			continue