mirror Discord deletions to X and bluesky

Deleting a message on the watched channel now deletes the posts it
produced. Making that possible reshapes how a message reaches an output:

  - OutputInterface takes an output.Post and returns an output.Ref, the
    identifier the platform gave the new post (tweet ID for X, URI and
    CID for bluesky). It also gained Delete(Ref).
  - The discord package no longer forwards raw MessageCreate values but
    a discord.Event carrying the kind of change, so the MessageDelete
    handler can use the same channel. Deleted messages arrive without an
    author, so only the channel ID can be filtered on.
  - The new store package remembers Discord message ID -> per output Ref.
    Set POST_STORE to a path to keep that mapping across restarts;
    without it the mapping is memory only and a restart makes older
    messages undeletable. It holds the most recent 1000 messages.

main.go grows a distributor type to hold the outputs and the store
rather than threading them through the event loop.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

output/stdout.go

@@ -11,11 +11,16 @@ func StdOutput() *stdoutput {
 	return &stdoutput{}
 }
 
-func (so *stdoutput) Write(str string, images []Image) error {
-	fmt.Println(str)
-	for _, img := range images {
+func (so *stdoutput) Write(post Post) (Ref, error) {
+	fmt.Println(post.Text)
+	for _, img := range post.Images {
 		fmt.Printf("[image: %s (%s, %d bytes)]\n", img.Filename, img.ContentType, len(img.Data))
 	}
+	return Ref{ID: post.Text}, nil
+}
+
+func (so *stdoutput) Delete(ref Ref) error {
+	fmt.Printf("[deleted: %s]\n", ref.ID)
 	return nil
 }