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/tools/result.go

@@ -30,6 +30,10 @@ type ToolResult struct {
 	// Err is the underlying error (not JSON serialized).
 	// Used for internal error handling and logging.
 	Err error `json:"-"`
+
+	// Media contains media store refs produced by this tool.
+	// When non-empty, the agent will publish these as OutboundMediaMessage.
+	Media []string `json:"media,omitempty"`
 }
 
 // NewToolResult creates a basic ToolResult with content for the LLM.
@@ -120,6 +124,19 @@ func UserResult(content string) *ToolResult {
 	}
 }
 
+// MediaResult creates a ToolResult with media refs for the user.
+// The agent will publish these refs as OutboundMediaMessage.
+//
+// Example:
+//
+//	result := MediaResult("Image generated successfully", []string{"media://abc123"})
+func MediaResult(forLLM string, mediaRefs []string) *ToolResult {
+	return &ToolResult{
+		ForLLM: forLLM,
+		Media:  mediaRefs,
+	}
+}
+
 // MarshalJSON implements custom JSON serialization.
 // The Err field is excluded from JSON output via the json:"-" tag.
 func (tr *ToolResult) MarshalJSON() ([]byte, error) {