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/output.go

@@ -6,7 +6,23 @@ type Image struct {
 	Filename    string
 }
 
+// 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.
+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
+}
+
+// Post is a single message to distribute.
+type Post struct {
+	Text   string
+	Images []Image
+}
+
 type OutputInterface interface {
-	Write(string, []Image) error
+	Write(Post) (Ref, error)
+	Delete(Ref) error
 	GetName() string
 }