feat(channels): auto-orchestrate Placeholder/Typing/Reaction via capability interfaces

Define PlaceholderCapable, TypingCapable, and ReactionCapable interfaces
and have BaseChannel.HandleMessage auto-detect and trigger all three as
independent pipelines on inbound messages. This replaces the scattered
manual orchestration code in each channel's handleMessage with a single
unified dispatch in the framework layer.

Changes:
- Add PlaceholderCapable interface to interfaces.go
- Add ReactionCapable + RecordReactionUndo to interfaces.go
- BaseChannel.HandleMessage auto-triggers Typing → Reaction → Placeholder
- Manager gains reactionUndos sync.Map with TTL janitor cleanup
- Telegram: extract SendPlaceholder from manual code, add StartTyping
- Discord: add SendPlaceholder + StartTyping
- Pico: add SendPlaceholder (uses Pico Protocol message.create)
- Slack: extract ReactToMessage from manual code
- OneBot: extract ReactToMessage, remove leaked pendingEmojiMsg sync.Map
- LINE: move group-chat guard into StartTyping, remove manual orchestration
- Config: add Placeholder to PicoConfig; remove from Slack/LINE/OneBot
  (no MessageEditor, so placeholder config was dead code)

pkg/channels/onebot/onebot.go

@@ -23,21 +23,20 @@ import (
 
 type OneBotChannel struct {
 	*channels.BaseChannel
-	config          config.OneBotConfig
-	conn            *websocket.Conn
-	ctx             context.Context
-	cancel          context.CancelFunc
-	dedup           map[string]struct{}
-	dedupRing       []string
-	dedupIdx        int
-	mu              sync.Mutex
-	writeMu         sync.Mutex
-	echoCounter     int64
-	selfID          int64
-	pending         map[string]chan json.RawMessage
-	pendingMu       sync.Mutex
-	lastMessageID   sync.Map
-	pendingEmojiMsg sync.Map
+	config        config.OneBotConfig
+	conn          *websocket.Conn
+	ctx           context.Context
+	cancel        context.CancelFunc
+	dedup         map[string]struct{}
+	dedupRing     []string
+	dedupIdx      int
+	mu            sync.Mutex
+	writeMu       sync.Mutex
+	echoCounter   int64
+	selfID        int64
+	pending       map[string]chan json.RawMessage
+	pendingMu     sync.Mutex
+	lastMessageID sync.Map
 }
 
 type oneBotRawEvent struct {
@@ -129,6 +128,22 @@ func (c *OneBotChannel) setMsgEmojiLike(messageID string, emojiID int, set bool)
 	}()
 }
 
+// ReactToMessage implements channels.ReactionCapable.
+// It adds an emoji reaction (ID 289) to group messages and returns an undo function.
+// Private messages return a no-op since reactions are only meaningful in groups.
+func (c *OneBotChannel) ReactToMessage(ctx context.Context, chatID, messageID string) (func(), error) {
+	// Only react in group chats
+	if !strings.HasPrefix(chatID, "group:") {
+		return func() {}, nil
+	}
+
+	c.setMsgEmojiLike(messageID, 289, true)
+
+	return func() {
+		c.setMsgEmojiLike(messageID, 289, false)
+	}, nil
+}
+
 func (c *OneBotChannel) Start(ctx context.Context) error {
 	if c.config.WSUrl == "" {
 		return fmt.Errorf("OneBot ws_url not configured")
@@ -1044,18 +1059,6 @@ func (c *OneBotChannel) handleMessage(raw *oneBotRawEvent) {
 
 	c.lastMessageID.Store(chatID, messageID)
 
-	if raw.MessageType == "group" && messageID != "" && messageID != "0" {
-		c.setMsgEmojiLike(messageID, 289, true)
-		c.pendingEmojiMsg.Store(chatID, messageID)
-		// Register emoji stop with Manager for outbound orchestration
-		if rec := c.GetPlaceholderRecorder(); rec != nil {
-			capturedMsgID := messageID
-			rec.RecordTypingStop("onebot", chatID, func() {
-				c.setMsgEmojiLike(capturedMsgID, 289, false)
-			})
-		}
-	}
-
 	senderInfo := bus.SenderInfo{
 		Platform:    "onebot",
 		PlatformID:  senderID,