Add `--additional-skills` option

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

Inside of copilot there's a `/skills add ~/path/to/skill`, but not a way to add them when launching copilot

### Proposed solution

Currently, users can specify additional map configs with `--additional-mcp-config`.

A similar feature would be nice for skills.

Currently, users can add additional skills inside of copilot with `/skills add ./skills`, but this requires adding additional skill directories with copilot already launched. And then, skills also need to be removed using `/skills remove ./skills`.

When launching copilot, it would be nice to have the ability to do something like:

```sh
copilot --additional-skills ~/study/skills --additional-skills ~/authoring/skills
```

Where additional skill directories are loaded for that session (not persisted).

This enables folks to write alias/launchers for copilot with both skills and MCP servers configured for custom tasks.

Then power users can role their own plugin system without copilot needing on internally for now.

### Example prompts or workflows

_No response_

### Additional context

_No response_