[Bug] Decomposing Writing (write) Type Tasks Results in All Think Type Sub-tasks, Halting Core Writing Progress

[wbok]

When using the StoryWritingNLPlanningEN prompt template for refined decomposition of novel writing tasks, it was found that the LLM's decomposition results for top-level Writing (write type) tasks do not meet expectations: all sub-tasks at the same level are decomposed into Think tasks, completely lacking Write type tasks, which makes it impossible to advance the core actual writing work.

Add the 10th rule to the Planning Tips in the system_message of the StoryWritingNLPlanningEN class to clearly constrain the decomposition logic:
10. When decomposing a writing (write) type task into sub-tasks, the sub-tasks in the same layer of the DAG cannot all be of the think type. At least one write type sub-task must be included in the same layer to ensure the advancement of the core writing work.