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/slack/adapter.ts

@@ -11,6 +11,7 @@ import type {
   Attachment,
   InboundMessage,
   OutboundMessage,
+  OutboundAttachment,
   ChannelAdapter,
   ChannelStatus,
 } from '../types.js';
@@ -152,6 +153,45 @@ export class SlackAdapter implements ChannelAdapter {
         });
       }
     }
+
+    // Send outbound attachments after text
+    if (message.attachments && message.attachments.length > 0) {
+      for (const attachment of message.attachments) {
+        await this.sendAttachment(channel, threadTs, attachment);
+      }
+    }
+  }
+
+  /** Upload and send a single outbound attachment via Slack's files.uploadV2 API. */
+  private async sendAttachment(
+    channel: string,
+    threadTs: string,
+    attachment: OutboundAttachment,
+  ): Promise<void> {
+    if (!this.app) return;
+
+    try {
+      if (attachment.data) {
+        await this.app.client.files.uploadV2({
+          channel_id: channel,
+          thread_ts: threadTs,
+          file: Buffer.from(attachment.data, 'base64'),
+          filename: attachment.filename ?? 'attachment',
+        });
+      } else if (attachment.url) {
+        // For URL-based attachments, share as a text message with the URL
+        await this.app.client.chat.postMessage({
+          channel,
+          text: attachment.url,
+          thread_ts: threadTs,
+        });
+      }
+    } catch (error) {
+      console.error(
+        `Slack: failed to send ${attachment.mimeType} attachment:`,
+        error instanceof Error ? error.message : 'Unknown error',
+      );
+    }
   }
 
   /** Resolve a Slack user ID to a display name, with caching. */
@@ -170,10 +210,10 @@ export class SlackAdapter implements ChannelAdapter {
   }
 
   /**
-   * Download image files from a Slack message and convert to base64 Attachments.
-   * Non-image files are skipped. Download errors are logged but don't crash the handler.
+   * Download media files from a Slack message and convert to base64 Attachments.
+   * Non-media files are skipped. Download errors are logged but don't crash the handler.
    */
-  private async extractImageAttachments(
+  private async extractMediaAttachments(
     files?: SlackMessageEvent['files'],
   ): Promise<Attachment[]> {
     if (!files || files.length === 0) return [];
@@ -181,8 +221,8 @@ export class SlackAdapter implements ChannelAdapter {
     const attachments: Attachment[] = [];
 
     for (const file of files) {
-      // Only process image files
-      if (!file.mimetype?.startsWith('image/')) continue;
+      // Only process image and audio files
+      if (!file.mimetype?.startsWith('image/') && !file.mimetype?.startsWith('audio/')) continue;
 
       const downloadUrl = file.url_private_download || file.url_private;
       if (!downloadUrl) continue;
@@ -259,8 +299,8 @@ export class SlackAdapter implements ChannelAdapter {
       ? await this.resolveUserName(message.user)
       : undefined;
 
-    // Extract image attachments from Slack file uploads
-    const attachments = await this.extractImageAttachments(message.files);
+    // Extract media attachments from Slack file uploads
+    const attachments = await this.extractMediaAttachments(message.files);
 
     // Detect reset command
     if (text === '!reset' || text === 'reset') {