Copilot Chat sessions not synchronized even though chat.sessionSync.enabled is true

- Copilot Chat Extension Version: 1.129.0
- VS Code Version: 1.129.0 (user setup)
- OS Version: Windows_NT x64 10.0.19045
- Feature (e.g. agent/edit/ask mode): session sync
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): not applicable
- Logs: --

Steps to Reproduce:
I'm using the Github Copilot chat in VS Code, and I have a Copilot Enterprise license. I have recently enabled the Session Sync option, which according to [this page](https://code.visualstudio.com/docs/agents/sessions/session-sync) should allow synchronizing sessions across VS Code and the Github Copilot Desktop app, but my local chat sessions in VS Code are not appearing anywhere.

1. Set `chat.sessionSync.enabled` and `github.copilot.chat.localIndex.enabled` to true in VS Code
2. Restart VS Code, run `/chronicle reindex` and wait a few hours (with an active internet connection)
3. Check the list of sessions on the Github Copilot Desktop app or on https://github.com/copilot/agents. The local sessions do not appear in any of the two.