feat: add WhatsApp and Email channel integrations

- WhatsApp Cloud API channel (Meta Business Platform)
  - Webhook verification, text/media messages, rate limiting
  - Phone number allowlist (empty=deny, *=allow, specific numbers)
  - Health check via API

- Email channel (IMAP/SMTP over TLS)
  - IMAP polling for inbound messages
  - SMTP sending with TLS
  - Sender allowlist (email, domain, wildcard)
  - HTML stripping, duplicate detection

Both implement ZeroClaw's Channel trait directly.
Includes inline unit tests.

src/channels/mod.rs

@@ -4,6 +4,7 @@ pub mod imessage;
 pub mod matrix;
 pub mod slack;
 pub mod telegram;
+pub mod whatsapp;
 pub mod traits;
 
 pub use cli::CliChannel;
@@ -12,6 +13,7 @@ pub use imessage::IMessageChannel;
 pub use matrix::MatrixChannel;
 pub use slack::SlackChannel;
 pub use telegram::TelegramChannel;
+pub use whatsapp::WhatsAppChannel;
 pub use traits::Channel;
 
 use crate::config::Config;