[Feature Request] Implement built-in `/create-*` skills similar to those in VS Code

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

I'd like to create skill, agent, hook, prompt, ... easily in Copilot CLI

### Proposed solution

Just like VS Code, it is very convenient.

### Example prompts or workflows

Simply type `/create-skill` to easily create a skill.

### Additional context

Start by tracing how Copilot CLI handles slash commands, using `/create-skill` as the first workflow to investigate. Define how the built-in commands should create skills, agents, hooks, and prompts, and consider the request complete when each requested artifact can be created through a corresponding command.

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