fix(harness): normalize structured content for titles (#1155)

* fix(harness): normalize structured content for titles

Flatten structured LangChain message content before prompting the title model so list/block payloads don't leak Python reprs into generated thread titles.

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

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

* Apply suggestions from code review

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

backend/docs/AUTO_TITLE_GENERATION.md

@@ -6,12 +6,14 @@
 
 ## 实现方式
 
-使用 `TitleMiddleware` 在 `after_agent` 钩子中：
+使用 `TitleMiddleware` 在 `after_model` 钩子中：
 1. 检测是否是首次对话（1个用户消息 + 1个助手回复）
 2. 检查 state 是否已有 title
 3. 调用 LLM 生成简洁的标题（默认最多6个词）
 4. 将 title 存储到 `ThreadState` 中（会被 checkpointer 持久化）
 
+TitleMiddleware 会先把 LangChain message content 里的结构化 block/list 内容归一化为纯文本，再拼到 title prompt 里，避免把 Python/JSON 的原始 repr 泄漏到标题生成模型。
+
 ## ⚠️ 重要：存储机制
 
 ### Title 存储位置