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/bluesky.go

@@ -67,6 +67,28 @@ func (bo *blueskyoutput) Write(post Post) (Ref, error) {
 		Langs:     []string{"ja"},
 	}
 
+	root := Ref{}
+	if post.ReplyTo != nil && post.ReplyTo.URI != "" {
+		parent := &atproto.RepoStrongRef{
+			Uri: post.ReplyTo.URI,
+			Cid: post.ReplyTo.CID,
+		}
+		// A post that is itself a reply carries the thread root; one that is
+		// not is the root of its own thread.
+		rootref := &atproto.RepoStrongRef{
+			Uri: post.ReplyTo.RootURI,
+			Cid: post.ReplyTo.RootCID,
+		}
+		if rootref.Uri == "" {
+			rootref = parent
+		}
+		feedpost.Reply = &bsky.FeedPost_ReplyRef{
+			Parent: parent,
+			Root:   rootref,
+		}
+		root = Ref{RootURI: rootref.Uri, RootCID: rootref.Cid}
+	}
+
 	if len(post.Images) > 0 {
 		embedimages := make([]*bsky.EmbedImages_Image, 0, len(post.Images))
 		for _, img := range post.Images {
@@ -96,7 +118,12 @@ func (bo *blueskyoutput) Write(post Post) (Ref, error) {
 		return Ref{}, recerr
 	}
 
-	return Ref{URI: record.Uri, CID: record.Cid}, nil
+	return Ref{
+		URI:     record.Uri,
+		CID:     record.Cid,
+		RootURI: root.RootURI,
+		RootCID: root.RootCID,
+	}, nil
 }
 
 func (bo *blueskyoutput) Delete(ref Ref) error {