refactor: tighten SubQuestions to 1-3 with Cantonese format hint

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/app/models/decompose.py

@@ -9,7 +9,7 @@ class SubQuestions(BaseModel):
     """
 
     questions: list[str] = Field(
-        description="2-5 simplified sub-questions, each focused on one aspect",
+        description="1-3 simplified sub-questions, each focused on one aspect並總格成以下格式 '標籤式主題: 具體提問/要求' ",
         min_length=1,
-        max_length=5,
+        max_length=3,
     )