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in frontend-repo's .github folder, then I can't be sure when backend folks update something on their end and I get outdated results.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

Beginne damit nachzuverfolgen, wie skillDirectories derzeit aus den Benutzereinstellungen gelesen wird und wie Repository-Einstellungen geladen werden. Vergleiche das Verhalten von .github/copilot/settings.json und .github/copilot/settings.local.json, einschließlich Skills in anderen Repositories und Unterverzeichnissen eines Monorepos. Erledigt ist die Aufgabe, wenn beide Dateien auf Repository-Ebene die exponierten Skill-Verzeichnisse anpassen können, ohne Skills zu kopieren.

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