config: add small_model to opencode.json

- Use Qwen 0.5B as small_model for fast title generation and summarization
- Same provider/model as primary (optimal for single-node local deployments)
- Reduces latency for lightweight tasks while keeping full model for complex work\n\nFixes #1039

Author: sbadakhc

opencode.json

@@ -67,5 +67,6 @@
       }
     }
   },
-  "model": "aixcl-local/Qwen/Qwen2.5-Coder-0.5B-Instruct"
+  "model": "aixcl-local/Qwen/Qwen2.5-Coder-0.5B-Instruct",
+  "small_model": "aixcl-local/Qwen/Qwen2.5-Coder-0.5B-Instruct"
 }