skillDirectories Does Not Take Effect at Repo Level

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

Working with skills spread across multiple repos or in subdirectories of mono-repos are currently not possible to include without a lot of copying around and cleaning up.

### Proposed solution

Support `skillDirectories` setting both in `.github/copilot/settings.json` and `.github/copilot/settings.local.json` so that we can customise which skills are exposed based on the type of work, the sub-path in a mono-repo and when working across multiple repos. All three scenarios are fairly common in a medium to large enterprise.

### Example prompts or workflows

Let's say I'm working on a frontend-repo and I notice that the backend is not adhering to the contract. I'd like from my frontend-repo to invoke the following skill that lives in backend-repo:
`/investigate-backlog to see if there's a bug on data path of controller Foo where it returns a string instead of a guid`

It wouldn't make sense for me to have it in my user settings since it's dependent upon the type of work I'm doing and the repo I'm currently using. If I duplicate the skill in frontend-repo's .github folder, then I can't be sure when backend folks update something on their end and I get outdated results.

### Additional context

A bit similar to the following issues but not the same IMO:
- #3688
- #3505

Commencez par retracer comment skillDirectories est actuellement lu depuis les paramètres utilisateur et comment les paramètres du dépôt sont chargés. Comparez le comportement de .github/copilot/settings.json et .github/copilot/settings.local.json, y compris les skills dans d'autres dépôts et les sous-répertoires d'un monorepo. La tâche est terminée lorsque les deux fichiers au niveau du dépôt peuvent personnaliser les répertoires de skills exposés sans copier de skills.

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