feat: add outbound attachment support with media.send tool

Introduces OutboundAttachment type on OutboundMessage, an
OutboundAttachmentCollector (push/drain pattern), and a media.send
tool that queues files for outbound delivery. Each channel adapter
(Telegram, Discord, Slack, WhatsApp) sends attachments after the
text reply. Includes 15 tests for collector and tool.

src/channels/types.ts

@@ -42,12 +42,26 @@ export interface InboundMessage {
   metadata?: Record<string, unknown>;
 }
 
+/** Attachment to send back via a channel adapter. */
+export interface OutboundAttachment {
+  /** MIME type (e.g. "image/png", "application/pdf"). */
+  mimeType: string;
+  /** Base64-encoded file content. */
+  data?: string;
+  /** URL to the file (alternative to data). */
+  url?: string;
+  /** Suggested filename. */
+  filename?: string;
+}
+
 /** Outbound message to send via a channel adapter. */
 export interface OutboundMessage {
   /** Response text (markdown). */
   text: string;
   /** Original message ID to reply to. */
   replyTo?: string;
+  /** File or image attachments to send with the response. */
+  attachments?: OutboundAttachment[];
   /** Platform-specific extras. */
   metadata?: Record<string, unknown>;
 }