feat: support single-assistant mem-reader

[CaralHsi]

Description

Summary: Add few-shot examples showing correct memory extraction when dialogue
contains only assistant messages, using "assistant suggested/recommended"
phrasing to prevent incorrect attribution to user.

添加 few-shot 示例，展示当对话仅包含助手消息时如何正确提取记忆。
使用"助手建议/推荐"等表述，避免将助手内容错误归因为用户陈述。

Docs Issue/PR: /

Reviewer: @(reviewer)

Checklist:

• I have performed a self-review of my own code | 我已自行检查了自己的代码
• I have commented my code in hard-to-understand areas | 我已在难以理解的地方对代码进行了注释
• I have added tests that prove my fix is effective or that my feature works | 我已添加测试以证明我的修复有效或功能正常
• I have created related documentation issue/PR in MemOS-Docs (if applicable) | 我已在 MemOS-Docs 中创建了相关的文档 issue/PR（如果适用）
• I have linked the issue to this PR (if applicable) | 我已将 issue 链接到此 PR（如果适用）
• I have mentioned the person who will review this PR | 我已提及将审查此 PR 的人