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>

tests/whatsapp_webhook_security.rs

@@ -0,0 +1,129 @@
+//! Integration tests for WhatsApp webhook signature verification.
+//!
+//! These tests validate that:
+//! 1. Webhooks with valid signatures are accepted
+//! 2. Webhooks with invalid signatures are rejected
+//! 3. Webhooks with missing signatures are rejected
+//! 4. Webhooks are rejected even if JSON is valid but signature is bad
+
+use hmac::{Hmac, Mac};
+use sha2::Sha256;
+
+/// Compute valid HMAC-SHA256 signature for a webhook payload
+fn compute_signature(app_secret: &str, body: &[u8]) -> String {
+    let mut mac = Hmac::<Sha256>::new_from_slice(app_secret.as_bytes()).unwrap();
+    mac.update(body);
+    let result = mac.finalize();
+    format!("sha256={}", hex::encode(result.into_bytes()))
+}
+
+#[test]
+fn whatsapp_signature_rejects_missing_sha256_prefix() {
+    let secret = "test_app_secret";
+    let body = b"test payload";
+    let bad_sig = "abc123"; // Missing sha256= prefix
+
+    assert!(!zeroclaw::gateway::verify_whatsapp_signature(
+        secret, body, bad_sig
+    ));
+}
+
+#[test]
+fn whatsapp_signature_rejects_invalid_hex() {
+    let secret = "test_app_secret";
+    let body = b"test payload";
+    let bad_sig = "sha256=not-valid-hex!!";
+
+    assert!(!zeroclaw::gateway::verify_whatsapp_signature(
+        secret, body, bad_sig
+    ));
+}
+
+#[test]
+fn whatsapp_signature_rejects_wrong_signature() {
+    let secret = "test_app_secret";
+    let body = b"test payload";
+    let bad_sig = "sha256=00112233445566778899aabbccddeeff";
+
+    assert!(!zeroclaw::gateway::verify_whatsapp_signature(
+        secret, body, bad_sig
+    ));
+}
+
+#[test]
+fn whatsapp_signature_accepts_valid_signature() {
+    let secret = "test_app_secret";
+    let body = b"test payload";
+    let valid_sig = compute_signature(secret, body);
+
+    assert!(zeroclaw::gateway::verify_whatsapp_signature(
+        secret, body, &valid_sig
+    ));
+}
+
+#[test]
+fn whatsapp_signature_rejects_tampered_body() {
+    let secret = "test_app_secret";
+    let original_body = b"original message";
+    let tampered_body = b"tampered message";
+
+    // Compute signature for original body
+    let sig = compute_signature(secret, original_body);
+
+    // Tampered body should be rejected even with valid-looking signature
+    assert!(!zeroclaw::gateway::verify_whatsapp_signature(
+        secret, tampered_body, &sig
+    ));
+}
+
+#[test]
+fn whatsapp_signature_rejects_wrong_secret() {
+    let correct_secret = "correct_secret";
+    let wrong_secret = "wrong_secret";
+    let body = b"test payload";
+
+    // Compute signature with correct secret
+    let sig = compute_signature(correct_secret, body);
+
+    // Wrong secret should reject the signature
+    assert!(!zeroclaw::gateway::verify_whatsapp_signature(
+        wrong_secret, body, &sig
+    ));
+}
+
+#[test]
+fn whatsapp_signature_rejects_empty_signature() {
+    let secret = "test_app_secret";
+    let body = b"test payload";
+
+    assert!(!zeroclaw::gateway::verify_whatsapp_signature(
+        secret, body, ""
+    ));
+}
+
+#[test]
+fn whatsapp_signature_different_secrets_produce_different_sigs() {
+    let secret1 = "secret_one";
+    let secret2 = "secret_two";
+    let body = b"same payload";
+
+    let sig1 = compute_signature(secret1, body);
+    let sig2 = compute_signature(secret2, body);
+
+    // Different secrets should produce different signatures
+    assert_ne!(sig1, sig2);
+
+    // Each signature should only verify with its own secret
+    assert!(zeroclaw::gateway::verify_whatsapp_signature(
+        secret1, body, &sig1
+    ));
+    assert!(!zeroclaw::gateway::verify_whatsapp_signature(
+        secret2, body, &sig1
+    ));
+    assert!(zeroclaw::gateway::verify_whatsapp_signature(
+        secret2, body, &sig2
+    ));
+    assert!(!zeroclaw::gateway::verify_whatsapp_signature(
+        secret1, body, &sig2
+    ));
+}