feat: add multimodal media pipeline for image support across all providers and channels

Widen Message.content from string to string | MessageContentPart[] to support
multimodal content. Add Attachment type to channel layer, media conversion
utilities, and image extraction to all channel adapters (Telegram, Discord,
Slack, WhatsApp). Update all model clients (Anthropic, OpenAI, Gemini, Bedrock)
to convert structured content to provider-specific formats. Fix downstream
consumers (tokens, compaction, TUI, local models) to handle the widened type
via getMessageText() helper.

src/channels/discord/adapter.ts

@@ -10,6 +10,7 @@ import { Client, GatewayIntentBits, Events } from 'discord.js';
 import type { Message as DiscordMessage } from 'discord.js';
 
 import type {
+  Attachment,
   InboundMessage,
   OutboundMessage,
   ChannelAdapter,
@@ -50,6 +51,20 @@ export class DiscordAdapter implements ChannelAdapter {
     this.config = config;
   }
 
+  /** Infer MIME type from URL if contentType is not provided. */
+  private _inferMimeTypeFromUrl(url: string): string | null {
+    const ext = url.split('.').pop()?.toLowerCase();
+    const mimeTypes: Record<string, string> = {
+      png: 'image/png',
+      jpg: 'image/jpeg',
+      jpeg: 'image/jpeg',
+      gif: 'image/gif',
+      webp: 'image/webp',
+      svg: 'image/svg+xml',
+    };
+    return mimeTypes[ext || ''] || null;
+  }
+
   /** Register the inbound message handler. Called by the registry before connect(). */
   onMessage(handler: (msg: InboundMessage) => void): void {
     this.messageHandler = handler;
@@ -159,6 +174,22 @@ export class DiscordAdapter implements ChannelAdapter {
     // Strip bot mention from the message text
     const text = message.content.replace(/<@!?\d+>/g, '').trim();
 
+    // ── Extract image 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/')) {
+          attachments.push({
+            mimeType,
+            url: attachment.url,
+            filename: attachment.name,
+            size: attachment.size,
+          });
+        }
+      }
+    }
+
     // ── Reset command ──
     if (text === '!reset' || text === 'reset') {
       this.messageHandler({
@@ -180,6 +211,7 @@ export class DiscordAdapter implements ChannelAdapter {
       senderId: message.channelId,
       senderName: message.author.username,
       text,
+      attachments: attachments.length > 0 ? attachments : undefined,
       timestamp: Date.now(),
     });
   }