feat: update merge prompt to reduce token cost

[gusye1234]

• Reduct the total insert token usage by 20~30%
• No Memory performance dropping observed.

Pasted new version of LOCOMO results here:

Mean Scores Per Category:
          bleu_score  f1_score  llm_score  count         type
category
1             0.3104    0.4385     0.6702    282   single_hop
2             0.4698    0.6209     0.8380    321     temporal
3             0.1896    0.2297     0.4792     96    multi_hop
4             0.3987    0.5118     0.7479    841  open_domain

Overall Mean Scores:
bleu_score    0.3843
f1_score      0.5035
llm_score     0.7357
dtype: float64