/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.

The relevant entry points are the /fork and /rewind commands. Reproduce the sequence in a clean repository with experimental mode enabled, then trace how forked sessions expose conversation history versus rewind history. Done means /rewind either rewinds the available last turn or clearly reports that rewind history is not copied.

Written by the indexing model from the issue text.