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/whatsapp/adapter.ts

@@ -7,11 +7,12 @@
  * Messages are chunked at 4096 chars (same as Telegram).
  */
 
-import { Client, LocalAuth } from 'whatsapp-web.js';
+import { Client, LocalAuth, MessageMedia } from 'whatsapp-web.js';
 import type {
   Attachment,
   InboundMessage,
   OutboundMessage,
+  OutboundAttachment,
   ChannelAdapter,
   ChannelStatus,
 } from '../types.js';
@@ -153,6 +154,38 @@ export class WhatsAppAdapter implements ChannelAdapter {
         await this.client.sendMessage(peerId, chunk);
       }
     }
+
+    // Send outbound attachments after text
+    if (message.attachments && message.attachments.length > 0) {
+      for (const attachment of message.attachments) {
+        await this.sendAttachment(peerId, attachment);
+      }
+    }
+  }
+
+  /** Send a single outbound attachment via WhatsApp using MessageMedia. */
+  private async sendAttachment(peerId: string, attachment: OutboundAttachment): Promise<void> {
+    if (!this.client) return;
+
+    try {
+      if (attachment.data) {
+        const media = new MessageMedia(
+          attachment.mimeType,
+          attachment.data,
+          attachment.filename,
+        );
+        await this.client.sendMessage(peerId, media);
+      } else if (attachment.url) {
+        // Download from URL and send as MessageMedia
+        const media = await MessageMedia.fromUrl(attachment.url);
+        await this.client.sendMessage(peerId, media);
+      }
+    } catch (error) {
+      console.error(
+        `WhatsApp: failed to send ${attachment.mimeType} attachment:`,
+        error instanceof Error ? error.message : 'Unknown error',
+      );
+    }
   }
 
   /** Internal: process an inbound WhatsApp message. */
@@ -211,17 +244,24 @@ export class WhatsAppAdapter implements ChannelAdapter {
 
     const senderName = message._data?.notifyName;
 
-    // Extract image attachments if the message has media
+    // Extract media attachments if the message has media
     const attachments: Attachment[] = [];
     if (message.hasMedia) {
       try {
         const media = await (message as any).downloadMedia();
-        if (media && typeof media.mimetype === 'string' && media.mimetype.startsWith('image/')) {
-          attachments.push({
-            mimeType: media.mimetype,
-            data: media.data,
-            filename: media.filename,
-          });
+        if (media && typeof media.mimetype === 'string') {
+          const mimeType = media.mimetype;
+          const isAudio = mimeType.startsWith('audio/');
+          const isImage = mimeType.startsWith('image/');
+          const isVoice = message.type === 'ptt';
+
+          if (isAudio || isImage || isVoice) {
+            attachments.push({
+              mimeType: mimeType,
+              data: media.data,
+              filename: media.filename,
+            });
+          }
         }
       } catch (error) {
         console.error(