Add `--additional-skills` option

[scaryrawr]

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:

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