/rewind reports nothing to rewind in forked sessions

[gfortaine]

describe the bug

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

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:

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.