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/types.go

@@ -24,3 +24,19 @@ type OutboundMessage struct {
 	ChatID  string `json:"chat_id"`
 	Content string `json:"content"`
 }
+
+// MediaPart describes a single media attachment to send.
+type MediaPart struct {
+	Type        string `json:"type"`                   // "image" | "audio" | "video" | "file"
+	Ref         string `json:"ref"`                    // media store ref, e.g. "media://abc123"
+	Caption     string `json:"caption,omitempty"`      // optional caption text
+	Filename    string `json:"filename,omitempty"`     // original filename hint
+	ContentType string `json:"content_type,omitempty"` // MIME type hint
+}
+
+// OutboundMediaMessage carries media attachments from Agent to channels via the bus.
+type OutboundMediaMessage struct {
+	Channel string      `json:"channel"`
+	ChatID  string      `json:"chat_id"`
+	Parts   []MediaPart `json:"parts"`
+}