Refresh session list without app restart (auto-detect sessions created outside the UI)

### Feature summary

_No response_

### What problem are you trying to solve?

# Refresh session list without app restart (auto-detect sessions created outside the UI)

## Problem

Sessions created outside the desktop app — e.g. by running `copilot` in a terminal — are persisted to the same local store the sidebar reads, but the sidebar doesn't reflect them until a full app restart. There's no in-app refresh affordance, so the only reliable way to surface a freshly-created external session is to quit and relaunch.

## Proposed solution

Either would solve it:

1. A manual **refresh** control on the project / session list that re-reads the store on demand, or
2. **Auto-refresh**: watch the session store (or listen for the registration event) and live-update the sidebar when a session is created/resumed outside the UI.

## Why it matters

Terminal `copilot` runs are a primary workflow for quick in-place tasks (PR reviews, support tickets) where a full worktree session is overkill. Having to restart the entire desktop app just to see those sessions in the sidebar is disruptive and easy to mistake for the session not being saved at all.

## Environment

GitHub Copilot desktop app on Windows.

### Proposed solution

_No response_

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

该 issue 未指定任何文件、测试或入口点。首先定位桌面侧边栏中的项目/会话列表和本地会话存储，然后确定由 terminal `copilot` 创建的会话是如何注册的。当新创建的外部会话无需重启应用即可通过手动刷新控件或自动更新显示时，即视为完成。

由索引模型根据 Issue 内容生成。