Collapsed project rows no longer show working / ready-to-review status (UX change from v1.0.20)

## Short summary
Since v1.0.20, collapsed project rows in the sidebar no longer reflect positive session states (working / in-progress, or completed / ready-to-review). The collapsed parent keeps a static project icon and only changes for attention states (needs input, interrupted, failed).

## Affected version or release
v1.0.23 (desktop shell 2.96.0). Behavior introduced in v1.0.20 (2026-07-10).

## Installation context
GitHub Copilot desktop app on Windows. Affects all projects/sessions shown in the sidebar.

## What happened?
Previously I could glance at a collapsed project row and immediately tell whether an agent was actively working, or whether a session had finished and was ready to review — without expanding the project. That at-a-glance rollup is what I relied on to triage across multiple projects.

As of v1.0.20 the collapsed project row shows a static project icon while sessions are working or completed/ready-to-review. Two changelog entries describe the new behavior:

- "Collapsed project rows in the sidebar keep their project icon for working, paused, or completed sessions, only replacing it when a session needs input, is interrupted, or has failed."
- "Collapsed repository groups in the sidebar now keep showing the repository icon when all sessions are completed or ready to merge, instead of looking like a single session."

I understand the intent (avoid a collapsed group looking like a single session), but the side effect is that the two most useful positive signals — working and ready-to-review — are no longer visible without expanding each project.

## Steps to reproduce
1. Have a project with one or more sessions in the sidebar, collapsed.
2. Start an agent working in a session (or let a session finish and become ready to review).
3. Observe the collapsed project row: the icon stays static and does not indicate the working or ready-to-review state. The only way to see progress or ready state is to expand the project and read the per-session rows.

## Expected behavior
Restore an at-a-glance rollup on collapsed project rows for positive states:
- A subtle indicator when any child session is actively working (spinner or dot), and
- A distinct indicator when any child session is completed / ready to review.

If the concern is collapsed groups being mistaken for single sessions, a good compromise would be a small badge/overlay on the project icon (rather than replacing the icon), or making the rollup behavior a user setting.

## Additional context
- Regression window: worked before v1.0.20; changed in v1.0.20 (2026-07-10); still present in v1.0.23.
- OS: Windows.

Commencez par reproduire le comportement des lignes de projet réduites dans la barre latérale de l’application de bureau, en utilisant à la fois des sessions enfants en cours et des sessions terminées/prêtes à être révisées. Suivez la manière dont les états des sessions enfants sont regroupés dans l’icône de projet réduite. C’est terminé lorsque les états positifs sont visibles d’un coup d’œil sans confondre le groupe de projet avec une seule session, et que le comportement existant des états d’attention est préservé.

Rédigé par le modèle d'indexation à partir du texte de l'issue.