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.

まず、デスクトップアプリのサイドバーにある折りたたまれたプロジェクト行の動作を、実行中と完了済み／レビュー準備完了の子セッションの両方を使って再現します。子セッションの状態が折りたたまれたプロジェクトアイコンにどのように集約されるかを追跡します。プロジェクトグループを単一のセッションと混同させることなく、肯定的な状態が一目で分かり、既存の注意状態の動作が維持されていれば完了です。

索引モデルが issue の本文から書いたものです。