fix: add-voice-transcription skill drops WhatsApp registerChannel call (#766)

The modify/src/channels/whatsapp.ts patch in the add-voice-transcription
skill was missing the registerChannel() call and its registry import.
When the three-way merge ran, this caused the WhatsApp channel to silently
skip registration on every service restart — messages were never received.

Added the missing import and registerChannel factory with a creds.json
guard, matching the pattern used by the add-whatsapp skill template.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/skills/add-voice-transcription/modify/src/channels/whatsapp.ts

@@ -20,6 +20,7 @@ import {
 import { logger } from '../logger.js';
 import { isVoiceMessage, transcribeAudioMessage } from '../transcription.js';
 import { Channel, OnInboundMessage, OnChatMetadata, RegisteredGroup } from '../types.js';
+import { registerChannel, ChannelOpts } from './registry.js';
 
 const GROUP_SYNC_INTERVAL_MS = 24 * 60 * 60 * 1000; // 24 hours
 
@@ -354,3 +355,12 @@ export class WhatsAppChannel implements Channel {
     }
   }
 }
+
+registerChannel('whatsapp', (opts: ChannelOpts) => {
+  const authDir = path.join(STORE_DIR, 'auth');
+  if (!fs.existsSync(path.join(authDir, 'creds.json'))) {
+    logger.warn('WhatsApp: credentials not found. Run /add-whatsapp to authenticate.');
+    return null;
+  }
+  return new WhatsAppChannel(opts);
+});