Create a Linux desktop entry when installing via Homebrew

### Feature summary

Create and register a Linux desktop entry for the GitHub Copilot App when it is installed through Homebrew.

### What problem are you trying to solve?

After installing the GitHub Copilot App with Homebrew on Ubuntu GNOME, the app is present as an AppImage but does not provide an obvious application launcher entry or desktop integration. Users must locate the AppImage manually and configure their own launcher or keyboard shortcut.

### Proposed solution

During installation, create a standard `.desktop` file in the user's applications directory, or otherwise integrate with the Linux desktop environment so that GitHub Copilot appears in the application menu and can be assigned a launcher shortcut normally.

The integration should remain valid after Homebrew upgrades to a new app version.

### Installation context

- OS: Ubuntu Linux with GNOME
- Installation method: Homebrew cask
- App version: 1.1.11
- Package format observed: `GitHub-Copilot-linux-x64.AppImage`

### Additional context

The app itself launches successfully; this request is specifically about desktop integration after Homebrew installation, not a request for a built-in global shortcut feature.

Start by locating the Homebrew cask definition for the GitHub Copilot App and reviewing how the AppImage is installed on Linux. Implement the desktop integration requested for Ubuntu GNOME, then verify that GitHub Copilot appears in the application menu and that the entry remains valid after a Homebrew upgrade.

索引モデルが issue の本文から書いたものです。