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>

src/providers/ollama.rs

@@ -88,10 +88,8 @@ impl Provider for OllamaProvider {
         let response = self.client.post(&url).json(&request).send().await?;
 
         if !response.status().is_success() {
-            let error = response.text().await?;
-            anyhow::bail!(
-                "Ollama error: {error}. Is Ollama running? (brew install ollama && ollama serve)"
-            );
+            let err = super::api_error("Ollama", response).await;
+            anyhow::bail!("{err}. Is Ollama running? (brew install ollama && ollama serve)");
         }
 
         let chat_response: ChatResponse = response.json().await?;