Add a `customization-creator` built-in skill

**Parent Issue:** #15327

Gemini CLI has a rich set of customization options: custom commands, hooks, agent skills, and full extensions. While documented, getting started with the boilerplate can be a hurdle for new developers.

We should create a built-in `customization-creator` skill. When activated, the agent should act as an expert on the CLI's own extensibility, helping users choose the right customization type and scaffolding the initial code for them.

**Goals:**
- Create the `customization-creator` built-in skill.
- Provide instructions so the agent understands the distinction between commands, hooks, skills, and extensions.
- Provide standard boilerplate templates that the agent can use to scaffold code for the user.
- Link the skill to our internal documentation so the agent has up-to-date API references.