feat(channels): add MediaSender optional interface for outbound media

Add outbound media sending capability so the agent can publish media
attachments (images, files, audio, video) through channels via the bus.

- Add MediaPart and OutboundMediaMessage types to bus
- Add PublishOutboundMedia/SubscribeOutboundMedia bus methods
- Add MediaSender interface discovered via type assertion by Manager
- Add media dispatch/worker in Manager with shared retry logic
- Extend ToolResult with Media field and MediaResult constructor
- Publish outbound media from agent loop on tool results
- Implement SendMedia for Telegram, Discord, Slack, LINE, OneBot, WeCom

pkg/bus/bus.go

@@ -10,17 +10,19 @@ import (
 var ErrBusClosed = errors.New("message bus closed")
 
 type MessageBus struct {
-	inbound  chan InboundMessage
-	outbound chan OutboundMessage
-	done     chan struct{}
-	closed   atomic.Bool
+	inbound       chan InboundMessage
+	outbound      chan OutboundMessage
+	outboundMedia chan OutboundMediaMessage
+	done          chan struct{}
+	closed        atomic.Bool
 }
 
 func NewMessageBus() *MessageBus {
 	return &MessageBus{
-		inbound:  make(chan InboundMessage, 100),
-		outbound: make(chan OutboundMessage, 100),
-		done:     make(chan struct{}),
+		inbound:       make(chan InboundMessage, 100),
+		outbound:      make(chan OutboundMessage, 100),
+		outboundMedia: make(chan OutboundMediaMessage, 100),
+		done:          make(chan struct{}),
 	}
 }
 
@@ -74,6 +76,31 @@ func (mb *MessageBus) SubscribeOutbound(ctx context.Context) (OutboundMessage, b
 	}
 }
 
+func (mb *MessageBus) PublishOutboundMedia(ctx context.Context, msg OutboundMediaMessage) error {
+	if mb.closed.Load() {
+		return ErrBusClosed
+	}
+	select {
+	case mb.outboundMedia <- msg:
+		return nil
+	case <-mb.done:
+		return ErrBusClosed
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+}
+
+func (mb *MessageBus) SubscribeOutboundMedia(ctx context.Context) (OutboundMediaMessage, bool) {
+	select {
+	case msg, ok := <-mb.outboundMedia:
+		return msg, ok
+	case <-mb.done:
+		return OutboundMediaMessage{}, false
+	case <-ctx.Done():
+		return OutboundMediaMessage{}, false
+	}
+}
+
 func (mb *MessageBus) Close() {
 	if mb.closed.CompareAndSwap(false, true) {
 		close(mb.done)