fix: correct Azure env var naming in .env.example

Author: DeveshParagiri

.env.example

@@ -15,6 +15,6 @@ OPENROUTER_API_KEY=sk-or-...
 # DeepSeek (from https://platform.deepseek.com/api_keys)
 DEEPSEEK_API_KEY=sk-...
 
-# Azure OpenAI (use one of these)
-AZURE_OPENAI_API_KEY=...
-# AZURE_API_KEY=...
+# Azure AI Foundry (AZURE_API_KEY is primary, AZURE_OPENAI_API_KEY is legacy fallback)
+AZURE_API_KEY=...
+# Also requires AZURE_ENDPOINT=https://<resource>.services.ai.azure.com/