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/stdout.go

@@ -12,6 +12,9 @@ func StdOutput() *stdoutput {
 }
 
 func (so *stdoutput) Write(post Post) (Ref, error) {
+	if post.ReplyTo != nil {
+		fmt.Printf("[reply to: %s]\n", post.ReplyTo.ID)
+	}
 	fmt.Println(post.Text)
 	for _, img := range post.Images {
 		fmt.Printf("[image: %s (%s, %d bytes)]\n", img.Filename, img.ContentType, len(img.Data))