Refresh/keep session in sync with external modifications (e.g. via VS Code)

### Feature summary

_No response_

### What problem are you trying to solve?

After implementing something in the app, it's common to e.g. open VS Code to tweak/review/continue manual work. The Github App session is visible inside VS Code, and can be continued there, but the changes aren't picked up back in the Github App; the latter must be restarted for it to reread the session continuation done outside in VS Code.

### Proposed solution

In an ideal world, the App would simply detect external session and show them; since that's probably complicated (two UIs working on the same copilot session), at least some way to refresh the session , similar to what a restart does would help.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

Note slight similarity with #1205, though that issue is more about picking up **new** sessions, whereas here it's about picking up continuations of existing/known sessions.

Der Issue nennt keine Dateien, Tests oder Einstiegspunkte. Beginne damit nachzuverfolgen, wie die GitHub App Copilot-Sitzungen liest und wie in VS Code erstellte Fortsetzungen nach einem Neustart der App sichtbar werden; abgeschlossen ist die Aufgabe, wenn eine bestehende Sitzung aktualisiert werden kann, um externe Änderungen anzuzeigen, ohne die App neu zu starten.

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