Please add support for multiple agent directories like you do for skills

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

Right now when using agents from multiple sources you have to manually copy them into the .copilot/agents directory, which is both tedious and error-prone.

### Proposed solution

When using agents from multiple sources it is much more convenient to simply add the various directories in which they reside to a list of agent directories within settings.json, just like you can with skills ("skillDirectories" in settings.json).

### Example prompts or workflows

It enables users to automatically combine the agents from several git repos into one combined list of agents without using copy commands.

### Additional context

_No response_

Start by tracing how settings.json reads skillDirectories and how agent files are currently discovered from .copilot/agents. Add equivalent support for a list of agent directories, then verify that agents from multiple repositories appear together without manual copying.

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