Implements the sandboxed-channel WIT interface for WhatsApp Cloud API:
- Webhook verification (GET with hub.mode=subscribe)
- Incoming message handling (POST webhooks)
- Outgoing responses via Graph API
- Parses WhatsApp webhook payload format
Built and tested in Docker sandbox with wasm32-wasip2 target.
Co-Authored-By: Claude Opus 4.5 <[email protected]>
