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.

首先，使用正在运行的子会话和已完成/准备好审核的子会话，复现桌面应用侧边栏中折叠项目行的行为。跟踪子会话状态如何汇总到折叠的项目图标中。完成的标准是：无需将项目组误认为单个会话，就能一眼看到积极状态，同时保留现有的注意状态行为。

由索引模型根据 Issue 内容生成。