main: change default model to qwen3.5:9b

qwen3:8b is too obtuse for agentic use; qwen3.5:9b is better at
instruction following and tool use at roughly the same size.

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

main.go

@@ -552,7 +552,7 @@ func main() {
 
 	modelName := os.Getenv("OLLIE_MODEL")
 	if modelName == "" {
-		modelName = "qwen3:8b"
+		modelName = "qwen3.5:9b"
 	}
 
 	// Resolve after backend.New() so that loadEnvFile has already run and