fix: clear media_paths after successful voice transcription\

\
  After transcribing a WhatsApp voice message, the .ogg file path          \
  remains in media_paths and gets appended as a [file: ...] tag.           \
  The LLM sees this tag and responds that it cannot process audio,          \
  even though the transcription already succeeded.

nanobot/channels/whatsapp.py

@@ -265,6 +265,7 @@ class WhatsAppChannel(BaseChannel):
                     transcription = await self.transcribe_audio(media_paths[0])
                     if transcription:
                         content = transcription
+                        media_paths = []
                         self.logger.info("Transcribed voice from {}: {}...", sender_id, transcription[:50])
                     else:
                         content = "[Voice Message: Transcription failed]"