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/interfaces.go

@@ -15,10 +15,27 @@ type MessageEditor interface {
 	EditMessage(ctx context.Context, chatID string, messageID string, content string) error
 }
 
+// ReactionCapable — channels that can add a reaction (e.g. 👀) to an inbound message.
+// ReactToMessage adds a reaction and returns an undo function to remove it.
+// The undo function MUST be idempotent and safe to call multiple times.
+type ReactionCapable interface {
+	ReactToMessage(ctx context.Context, chatID, messageID string) (undo func(), err error)
+}
+
+// PlaceholderCapable — channels that can send a placeholder message
+// (e.g. "Thinking... 💭") that will later be edited to the actual response.
+// The channel MUST also implement MessageEditor for the placeholder to be useful.
+// SendPlaceholder returns the platform message ID of the placeholder so that
+// Manager.preSend can later edit it via MessageEditor.EditMessage.
+type PlaceholderCapable interface {
+	SendPlaceholder(ctx context.Context, chatID string) (messageID string, err error)
+}
+
 // PlaceholderRecorder is injected into channels by Manager.
 // Channels call these methods on inbound to register typing/placeholder state.
 // Manager uses the registered state on outbound to stop typing and edit placeholders.
 type PlaceholderRecorder interface {
 	RecordPlaceholder(channel, chatID, placeholderID string)
 	RecordTypingStop(channel, chatID string, stop func())
+	RecordReactionUndo(channel, chatID string, undo func())
 }