/remote on fails in VS Code 1.137+ with "Failed to set up remote session" and "Session is not an adoptable legacy chat"

- Copilot Chat Extension Version: 0.66.0
- VS Code Version: 1.138.0 (also happens on 1.137.0)
- OS Version: Windows 11 Build 26200.9457
- Feature (e.g. agent/edit/ask mode): Copilot in any mode
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): All

Steps to Reproduce:
1. Upgrade from v136 to v137 or v138.
2. Start a Copilot session in the VS Code Github Chat (not Agents window)
3. type "/remote on"

"/remote on" is failing. It works fine in v136 and before but is broken in v137 and v138 giving the error below. This is happening on two separate computers and multiple repositories. I have verified that I am signed into Github and all other functions work as expected. Rolling back to v136 fixes the issue.

Results in this error:
Error: (sendFailed) Error: Request session.commands.invoke failed with message: Failed to set up remote session. Ensure the working directory is a GitHub repository and you are authenticated.

Developer Console shows error:
Session is not an adoptable legacy chat:
copilotcli:/untitled-...

I have verified the following:
Authentication succeeds: Logged in as GitHub account
Copilot token acquired
Interactive authentication succeeded for https://api.github.com
Fresh profile reproduces issue.

No files or tests are named. First reproduce the regression by comparing VS Code 1.136 with 1.137 or 1.138, using GitHub Chat and `/remote on`, then inspect the Developer Console output and the session command entry point. Done means `/remote on` establishes a remote session without the reported errors while authentication remains successful.

Written by the indexing model from the issue text.