Add global listener for GitHub Pull Request extension installation

[Copilot]

Addresses feedback from #1642 to detect GHPR extension installation via two paths: explicit install flow and manual user installation.

Changes

• Global listener: Added persistent vscode.extensions.onDidChange listener that detects when GHPR extension is installed by any means and updates context key
• Event-based explicit install: Replaced polling with onDidChange event to wait for extension installation, maintaining reload notification for explicit install case only

Implementation

// Global listener monitors all extension changes
this._register(vscode.extensions.onDidChange(() => {
    if (this.isPullRequestExtensionInstalled()) {
        vscode.commands.executeCommand('setContext', prExtensionInstalledContextKey, true);
    }
}));

// Explicit install uses event listener, shows reload notification
await new Promise<void>((resolve, reject) => {
    const listener = vscode.extensions.onDidChange(() => {
        if (this.isPullRequestExtensionInstalled()) {
            listener.dispose();
            // Show reload notification...
            resolve();
        }
    });
});

The global listener catches manual installations silently, while the explicit install flow shows the reload prompt.

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