Changes tab does not update after the session's PR is merged

Describe the bug

In the Copilot desktop app (Tauri), a session's Changes tab does not appear to refresh after the session's pull request is merged on GitHub. The tab continues to reflect the pre-merge state instead of updating to the new reality (merged branch / updated base).

Note: this report is about the desktop app UI, not the CLI itself — filed here as the reachable public tracker; please reroute if there is a dedicated app repo.

Affected version

Desktop app ~1.1.14 (inferred from ~/.copilot/data.db.pre-update-backup-1.1.14-*); embedded CLI reports version 1.0.80.

Steps to reproduce the behavior

1. Open a worktree-backed project session on a feature branch with changes.
2. Create a PR from the session and merge it on GitHub while the session stays open.
3. Return to the session and open the Changes tab.
4. Observe that the view still shows the pre-merge state; it does not update after the merge.

Expected behavior

After the session's PR merges, the Changes tab refreshes — e.g., recomputes the diff against the updated default branch (or shows an appropriate post-merge state) without requiring manual intervention.

Additional context

• OS: macOS (Darwin, ARM).
• Session type: worktree-backed, branch created and pushed by the session; PR merged via the GitHub web UI.
• Possibly related: inside the session, the branch is not auto-updated after merge either (git fetch/merge origin/main had to be run manually), so the tab may be diffing against a stale merge-base.
• Reported by the session's user mid-workflow and relayed by the agent; happy to pull logs (--log-level/--log-file) if you tell me which ones help.

Start with the Tauri desktop app's Changes tab and the session pull-request/branch update path, using the reported worktree-backed macOS setup to reproduce the stale view after merging on GitHub. Trace when the tab refreshes and how the merge-base is updated; done means the tab shows the post-merge state without requiring manual fetch or merge commands.