Conversation branching no longer works after an update

**提交人**: 匿名
**客户端版本**: 0.1.72

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### Summary

Creating a new conversation branch no longer works after a Cindy update.

### Expected behavior

The user should be able to create a new branch from an existing conversation while keeping the original conversation available.

### Observed behavior

The user cannot create a new conversation branch. This capability worked before the update.

### Impact

The user cannot explore an alternative line of reasoning without altering or duplicating the original conversation manually.
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## 提交时的任务环境

仅代表提交时快照,不一定是故障环境。OS 来自提交客户端本机,不含 SSH 远端主机;Harness / 模型来自当前任务。与运行环境无关的反馈可忽略本段。
**版本区域**: CN
**OS**: win32 x64 (10.0.26200)
**Harness**: Codex
**Model ID**: ` gpt-5.6-sol `
**界面语言**: zh-CN

No file or test is identified in the report. First reproduce branching on version 0.1.72 and trace the conversation-branching entry point; done means a new branch is created while the original conversation remains available, with a regression check for that behavior.

Written by the indexing model from the issue text.