Add /create-skill command

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

add the **`/create-skill`** command like [that in VSCode Copilot Chat](https://code.visualstudio.com/docs/copilot/customization/agent-skills#_generate-a-skill-with-ai), which directly makes AI know about and correctly create agent skill, without tedious explanations (e.g., what an agent skill is, etc.) from the user.

### Proposed solution

The user can just specify his needs and ask the agent to create some skill files.
It will make creating skill files easier and more intelligent, definitely benefiting Copilot-Cli and its users.
Also, both Codex and Claude Code have this feature.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Inizia esaminando i punti di ingresso dei comandi esistenti di Copilot CLI e la documentazione collegata di VSCode Copilot Chat per capire come dovrebbe funzionare /create-skill. Confronta le funzionalità di Codex e Claude Code indicate, quindi definisci l’input del comando, i file skill generati e i criteri di completamento prima dell’implementazione.

Scritto dal modello di indicizzazione a partire dal testo della issue.