Plugin Update Alerts

### Describe the feature or problem you'd like to solve

When a newer version of a plugin is available, I'd like to be alerted

### Proposed solution

When starting Copilot, automatically check my installed plugins for newer versions and alert me with the option to easily update the plugins that are out of date.

### Example prompts or workflows

_No response_

### Additional context

_No response_

No files, tests, or entry points are named. Start by locating the Copilot startup flow and installed-plugin management entry points, then determine how update checks and user prompts are handled; done means outdated plugins are detected at startup, users are alerted, and they can update them easily.

Rédigé par le modèle d'indexation à partir du texte de l'issue.