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/output.go

@@ -7,18 +7,28 @@ type Image struct {
 }
 
 // Ref identifies a post an output has already published so it can later be
-// deleted. The fields are output specific; only the ones the publishing
-// output filled in are meaningful to it.
+// replied to or deleted. The fields are output specific; only the ones the
+// publishing output filled in are meaningful to it.
 type Ref struct {
-	ID  string `json:"id,omitempty"`  // twitter: tweet ID
-	URI string `json:"uri,omitempty"` // bluesky: at:// URI of the record
-	CID string `json:"cid,omitempty"` // bluesky: record CID
+	ID      string `json:"id,omitempty"`  // twitter: tweet ID
+	URI     string `json:"uri,omitempty"` // bluesky: at:// URI of the record
+	CID     string `json:"cid,omitempty"` // bluesky: record CID
+	RootURI string `json:"rooturi,omitempty"`
+	RootCID string `json:"rootcid,omitempty"`
+}
+
+// IsZero reports whether the ref points at nothing.
+func (r Ref) IsZero() bool {
+	return r == Ref{}
 }
 
 // Post is a single message to distribute.
 type Post struct {
 	Text   string
 	Images []Image
+	// ReplyTo is the Ref this same output returned for the post being
+	// replied to, or nil for a top level post.
+	ReplyTo *Ref
 }
 
 type OutputInterface interface {