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/agent/loop.go

@@ -728,6 +728,19 @@ func (al *AgentLoop) runLLMIteration(
 					})
 			}
 
+			// If tool returned media refs, publish them as outbound media
+			if len(toolResult.Media) > 0 && opts.SendResponse {
+				parts := make([]bus.MediaPart, 0, len(toolResult.Media))
+				for _, ref := range toolResult.Media {
+					parts = append(parts, bus.MediaPart{Ref: ref})
+				}
+				al.bus.PublishOutboundMedia(ctx, bus.OutboundMediaMessage{
+					Channel: opts.Channel,
+					ChatID:  opts.ChatID,
+					Parts:   parts,
+				})
+			}
+
 			// Determine content for LLM based on tool result
 			contentForLLM := toolResult.ForLLM
 			if contentForLLM == "" && toolResult.Err != nil {