feat: implement pembantu whatsapp

[rayenking]

Added a WhatsApp bot implementation using whatsapp-rust.

Highlights:

• Pairing Code Login: Support for login via pairing code (useful if you can't scan QRs).
• AI Integration: The bot can now respond to messages using an AI backend from pembantu_core.
• Command Handling: Basic command parsing logic is in place (e.g., try ping).
• Docs: Added a quick setup guide in the README.

How to test:

1. Run cargo +nightly run -p pembantu_whatsapp.
2. Enter your phone number with + prefix and use the 8-digit code to pair in your WA mobile app.
3. Send a message to the bot to see if the AI responds!