Agent feedback sent to parent session instead of fork

Does this issue occur when all extensions are disabled?: Yes/No

- VS Code Version: latest Insiders
- OS Version: Windows

Steps to Reproduce:

1. In Agents window, fork an existing session (has no file changes)
2. In forked session, let the agent make file changes
3. After completion, add inline feedback on the agent changes and submit them
4. The parent session receives the `/act-on-feedback` command instead of the forked session

Start in the Agents window using the four reproduction steps: fork a session, make file changes, submit inline feedback, and observe which session receives `/act-on-feedback`. Trace how the forked session and feedback submission are associated, then verify that the command reaches the fork rather than the parent.

Written by the indexing model from the issue text.