/rewind reports nothing to rewind in forked sessions

### describe the bug

after forking a session with `/fork`, running `/rewind` inside the forked session reports:

```text
nothing to rewind yet. start a conversation first.
```

this happens even though the forked session visibly contains the copied conversation history from the original session.

### steps to reproduce

1. create a clean git repository.
2. start copilot cli with experimental mode enabled.
3. ask copilot to make a small file change.
4. run `/fork`.
5. in the forked session, run `/rewind`.

### expected behavior

`/rewind` should either rewind the last turn available in the forked session, or clearly explain that rewind history is intentionally not copied into forked sessions.

### actual behavior

the forked session shows:

```text
nothing to rewind yet. start a conversation first.
```

### notes

i reproduced this locally with copilot cli 1.0.46 in a temporary git repository. this was originally noticed by @seanf while discussing #2058.

相關入口是 /fork 和 /rewind 命令。在啟用實驗模式的乾淨儲存庫中重現該序列，然後追蹤 fork 工作階段如何公開對話記錄與 rewind 記錄。完成的標準是：/rewind 要麼回退可用的上一輪，要麼明確回報 rewind 記錄未被複製。

由索引模型根據 Issue 內容生成。