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/onboard/wizard.rs

@@ -1700,8 +1700,8 @@ fn setup_channels() -> Result<ChannelsConfig> {
                     access_token: access_token.trim().to_string(),
                     phone_number_id: phone_number_id.trim().to_string(),
                     verify_token: verify_token.trim().to_string(),
-                    allowed_numbers,
                     app_secret: None, // Can be set via ZEROCLAW_WHATSAPP_APP_SECRET env var
+                    allowed_numbers,
                 });
             }
             6 => {