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/discord/adapter.ts

@@ -6,13 +6,14 @@
  * Messages are chunked at Discord's 2000-char limit.
  */
 
-import { Client, GatewayIntentBits, Events } from 'discord.js';
+import { Client, GatewayIntentBits, Events, AttachmentBuilder } from 'discord.js';
 import type { Message as DiscordMessage } from 'discord.js';
 
 import type {
   Attachment,
   InboundMessage,
   OutboundMessage,
+  OutboundAttachment,
   ChannelAdapter,
   ChannelStatus,
 } from '../types.js';
@@ -121,7 +122,7 @@ export class DiscordAdapter implements ChannelAdapter {
     }
 
     const text = message.text;
-    const sendable = channel as { send: (content: string) => Promise<unknown> };
+    const sendable = channel as { send: (content: string | Record<string, unknown>) => Promise<unknown> };
 
     if (text.length <= 2000) {
       await sendable.send(text);
@@ -131,6 +132,30 @@ export class DiscordAdapter implements ChannelAdapter {
         await sendable.send(chunk);
       }
     }
+
+    // Send outbound attachments after text
+    if (message.attachments && message.attachments.length > 0) {
+      const files = message.attachments
+        .filter((a) => a.data || a.url)
+        .map((a) => this.buildDiscordAttachment(a));
+
+      if (files.length > 0) {
+        await sendable.send({ files });
+      }
+    }
+  }
+
+  /** Build a discord.js AttachmentBuilder from an OutboundAttachment. */
+  private buildDiscordAttachment(attachment: OutboundAttachment): AttachmentBuilder {
+    if (attachment.data) {
+      return new AttachmentBuilder(Buffer.from(attachment.data, 'base64'), {
+        name: attachment.filename ?? 'attachment',
+      });
+    }
+    // URL-based attachment
+    return new AttachmentBuilder(attachment.url!, {
+      name: attachment.filename ?? 'attachment',
+    });
   }
 
   /** Internal: process an inbound Discord message. */
@@ -174,12 +199,12 @@ export class DiscordAdapter implements ChannelAdapter {
     // Strip bot mention from the message text
     const text = message.content.replace(/<@!?\d+>/g, '').trim();
 
-    // ── Extract image attachments ──
+    // ── Extract media attachments ──
     const attachments: Attachment[] = [];
     if (message.attachments && message.attachments.size > 0) {
       for (const attachment of message.attachments.values()) {
         const mimeType = attachment.contentType || this._inferMimeTypeFromUrl(attachment.url);
-        if (mimeType && mimeType.startsWith('image/')) {
+        if (mimeType && (mimeType.startsWith('image/') || mimeType.startsWith('audio/'))) {
           attachments.push({
             mimeType,
             url: attachment.url,