fix: cliproxy base_url 경로 수정 + Sonnet 모델 설정

- base_url: https://cliproxy.gru.farm/api/provider/claude
- model: claude-sonnet-4-6

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

config/engine.json

@@ -21,8 +21,8 @@
       },
       "claude": {
         "api_key_env": "CLIPROXY_API_KEY",
-        "base_url": "https://cliproxy.gru.farm",
-        "model": "claude-opus-4-6",
+        "base_url": "https://cliproxy.gru.farm/api/provider/claude",
+        "model": "claude-sonnet-4-6",
         "max_tokens": 4096,
         "temperature": 0.7
       },