Copilot App: session name keeps reverting to a linked (merged) PR's title after an explicit rename #5158

Describe the bug

In the Copilot app, once a session has a pull request linked to it because the agent opened that PR from the session, the app replaces the session's name with the PR title. If the session is renamed afterwards, the new name doesn't stick: some time later it is reset to the PR title again. This keeps happening after the PR has been merged, with no new activity on the PR.

There is no way to unlink a merged PR from a session, so a long-running session that once opened a small side PR keeps losing its name.

Steps to reproduce

1. Open a local (branch) session in a repository and give it a name, e.g. "My topic".
2. In that session, have the agent open a small pull request with the app's built-in create-pull-request tool. The session name changes to the PR title.
3. Merge the PR.
4. Rename the session back to "My topic" (via the agent's rename-session tool with force, which reports success).
5. Keep working in the session. Some time later, the session name is the merged PR's title again.

Observed twice in one session: the explicit rename reverted to the PR title within about 30 minutes the first time and within about 4.5 hours the second time. Nothing happened on the PR in between; it had been merged for hours.

Expected behavior

An explicit rename, by the user or by the agent on the user's request, persists. The linked PR's title should at most be the default name for a session that was never named. Alternatively, it should be possible to unlink a PR from a session.

Actual behavior

The linked PR's title periodically overwrites the explicit session name, even for a merged PR.

Additional context

PRs opened from the same session with the gh CLI instead of the app's PR tool don't link to the session and don't affect its name.