fix: add WhatsApp webhook signature verification (X-Hub-Signature-256)

Closes #51

- Add HMAC-SHA256 signature verification for WhatsApp webhooks
- Prevents message spoofing attacks (CWE-345)
- Add whatsapp_app_secret config field with ZEROCLAW_WHATSAPP_APP_SECRET env override
- Add 13 comprehensive unit tests

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

Cargo.lock

@@ -396,6 +396,7 @@ checksum = "9ed9a281f7bc9b7576e61468ba615a66a5c8cfdff42420a70aa82701a3b1e292"
 dependencies = [
  "block-buffer",
  "crypto-common",
+ "subtle",
 ]
 
 [[package]]
@@ -644,6 +645,21 @@ version = "0.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
 
+[[package]]
+name = "hex"
+version = "0.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7f24254aa9a54b5c858eaee2f5bccdb46aaf0e486a595ed5fd8f86ba55232a70"
+
+[[package]]
+name = "hmac"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6c49c37c09c17a53d937dfbb742eb3a961d65a994e6bcdcf37e7399d0cc8ab5e"
+dependencies = [
+ "digest",
+]
+
 [[package]]
 name = "hostname"
 version = "0.4.2"
@@ -2553,6 +2569,8 @@ dependencies = [
  "dialoguer",
  "directories",
  "futures-util",
+ "hex",
+ "hmac",
  "hostname",
  "http-body-util",
  "reqwest",