feat: add WhatsApp and Email channel integrations

Adds WhatsApp (Cloud API) and Email (IMAP/SMTP) as new channels.

**WhatsApp Channel (`src/channels/whatsapp.rs`)**
- Meta Business Cloud API v18.0
- Webhook verification (hub.challenge flow)
- Inbound text, image, and document messages
- Outbound text via Cloud API
- Phone number allowlist with rate limiting
- Health check against API
- X-Hub-Signature-256 webhook signature verification

**Email Channel (`src/channels/email_channel.rs`)**
- IMAP over TLS (rustls) for inbound polling
- SMTP via lettre with STARTTLS for sending
- Sender allowlist (specific address, @domain, * wildcard)
- HTML stripping for clean text extraction
- Duplicate message detection
- Configurable poll interval and folder

All 906 tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/config/schema.rs

@@ -763,7 +763,8 @@ pub struct WhatsAppConfig {
     pub phone_number_id: String,
     /// Webhook verify token (you define this, Meta sends it back for verification)
     pub verify_token: String,
-    /// App secret for webhook signature verification (X-Hub-Signature-256)
+    /// App secret from Meta Business Suite (for webhook signature verification)
+    /// Can also be set via `ZEROCLAW_WHATSAPP_APP_SECRET` environment variable
     #[serde(default)]
     pub app_secret: Option<String>,
     /// Allowed phone numbers (E.164 format: +1234567890) or "*" for all
@@ -1488,7 +1489,7 @@ channel_id = "C123"
             access_token: "tok".into(),
             phone_number_id: "12345".into(),
             verify_token: "verify".into(),
-            app_secret: None,
+            app_secret: Some("secret123".into()),
             allowed_numbers: vec!["+1".into()],
         };
         let toml_str = toml::to_string(&wc).unwrap();