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/bluesky.go

@@ -3,7 +3,9 @@ package output
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
+	"strings"
 	"time"
 
 	"github.com/bluesky-social/indigo/api/atproto"
@@ -13,6 +15,8 @@ import (
 	"github.com/bluesky-social/indigo/xrpc"
 )
 
+const blueskyCollection = "app.bsky.feed.post"
+
 type blueskyoutput struct {
 	identifier string
 	password   string
@@ -27,7 +31,8 @@ func BlueskyOutput(identifier string, password string) *blueskyoutput {
 	return blueskyoutput
 }
 
-func (bo *blueskyoutput) Write(str string, images []Image) error {
+// session logs in and returns a client authenticated as the configured account.
+func (bo *blueskyoutput) session() (*xrpc.Client, error) {
 	cli := &xrpc.Client{
 		Host: "https://bsky.social",
 	}
@@ -39,7 +44,7 @@ func (bo *blueskyoutput) Write(str string, images []Image) error {
 
 	output, err := atproto.ServerCreateSession(context.TODO(), cli, input)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	cli.Auth = &xrpc.AuthInfo{
 		AccessJwt:  output.AccessJwt,
@@ -47,26 +52,34 @@ func (bo *blueskyoutput) Write(str string, images []Image) error {
 		Handle:     output.Handle,
 		Did:        output.Did,
 	}
+	return cli, nil
+}
 
-	post := &bsky.FeedPost{
-		Text:      str,
+func (bo *blueskyoutput) Write(post Post) (Ref, error) {
+	cli, err := bo.session()
+	if err != nil {
+		return Ref{}, err
+	}
+
+	feedpost := &bsky.FeedPost{
+		Text:      post.Text,
 		CreatedAt: time.Now().Format(util.ISO8601),
 		Langs:     []string{"ja"},
 	}
 
-	if len(images) > 0 {
-		embedimages := make([]*bsky.EmbedImages_Image, 0, len(images))
-		for _, img := range images {
+	if len(post.Images) > 0 {
+		embedimages := make([]*bsky.EmbedImages_Image, 0, len(post.Images))
+		for _, img := range post.Images {
 			blob, err := atproto.RepoUploadBlob(context.TODO(), cli, bytes.NewReader(img.Data))
 			if err != nil {
-				return fmt.Errorf("uploading %s: %w", img.Filename, err)
+				return Ref{}, fmt.Errorf("uploading %s: %w", img.Filename, err)
 			}
 			embedimages = append(embedimages, &bsky.EmbedImages_Image{
 				Alt:   img.Filename,
 				Image: blob.Blob,
 			})
 		}
-		post.Embed = &bsky.FeedPost_Embed{
+		feedpost.Embed = &bsky.FeedPost_Embed{
 			EmbedImages: &bsky.EmbedImages{
 				Images: embedimages,
 			},
@@ -74,13 +87,47 @@ func (bo *blueskyoutput) Write(str string, images []Image) error {
 	}
 
 	Recordinput := &atproto.RepoCreateRecord_Input{
-		Collection: "app.bsky.feed.post",
+		Collection: blueskyCollection,
 		Repo:       cli.Auth.Did, // "matope.bsky.social" のDID
-		Record:     &lexutil.LexiconTypeDecoder{Val: post},
+		Record:     &lexutil.LexiconTypeDecoder{Val: feedpost},
+	}
+	record, recerr := atproto.RepoCreateRecord(context.TODO(), cli, Recordinput)
+	if recerr != nil {
+		return Ref{}, recerr
 	}
-	_, recerr := atproto.RepoCreateRecord(context.TODO(), cli, Recordinput)
 
-	return recerr
+	return Ref{URI: record.Uri, CID: record.Cid}, nil
+}
+
+func (bo *blueskyoutput) Delete(ref Ref) error {
+	rkey, err := recordKey(ref.URI)
+	if err != nil {
+		return err
+	}
+
+	cli, err := bo.session()
+	if err != nil {
+		return err
+	}
+
+	_, err = atproto.RepoDeleteRecord(context.TODO(), cli, &atproto.RepoDeleteRecord_Input{
+		Collection: blueskyCollection,
+		Repo:       cli.Auth.Did,
+		Rkey:       rkey,
+	})
+	return err
+}
+
+// recordKey pulls the rkey out of an at://did/collection/rkey URI.
+func recordKey(uri string) (string, error) {
+	if uri == "" {
+		return "", errors.New("no record URI to delete")
+	}
+	idx := strings.LastIndex(uri, "/")
+	if idx < 0 || idx == len(uri)-1 {
+		return "", fmt.Errorf("malformed record URI %q", uri)
+	}
+	return uri[idx+1:], nil
 }
 
 func (bo *blueskyoutput) GetName() string {