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_

The issue names no files, tests, or entry points. Start by locating the desktop sidebar's project/session list and the local session store, then determine how sessions created by terminal `copilot` are registered. Done means a newly created external session appears without restarting the app, through either a manual refresh control or automatic updating.

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