Agents window: unable to create PR when remote is not origin

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

- VS Code Version: latest Insiders (1.136)
- OS Version: Windows

When the remote repository is not called "origin", the create PR functionality in the Agents window silently fails and no PR is created.

Log output:

```text
2026-08-28 10:45:55.588 [info] [AgentHostPullRequestOperationHandler] Pushing branch ntrogh/insider-release-notes to origin for session copilotcli:/16a51721-0272-467b-8971-18c3c65c3932
2026-08-28 10:45:55.715 [warning] [agentHostGitService] > git rev-parse --abbrev-ref ntrogh/insider-release-notes@{upstream} failed; full stderr:
fatal: no upstream configured for branch 'ntrogh/insider-release-notes'

2026-08-28 10:45:56.016 [warning] [agentHostGitService] > git push --set-upstream origin ntrogh/insider-release-notes failed; full stderr:
fatal: 'origin' does not appear to be a git repository
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

2026-08-28 10:45:56.019 [error] [ProtocolServer] Request 'invokeChangesetOperation' failed Failed to push branch 'ntrogh/insider-release-notes': git push exited with code 128: and the repository exists.
```