Add a guided journey for adding marketplace/plugins

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

Improve ergonics and comparison to CC

### Proposed solution

CP and CC offer sharply different experiences here. This is a bit of the goldilocks problem. CP is too cold and CC is too warm. If marketplaces and plugins are the big new thing, I think there is room to do more investment here.

- https://github.com/richlander/dotnet-skills/issues/1
- https://github.com/richlander/dotnet-skills/issues/2

The Claude Code experience was vastly super for learning how this whole marketplace concept works. I was quite confused at first. I went with CC first beause CP was too confusing. Once I learned that I sort of liked the CP experience better, as in "power mode". The proble with powermode is that it is basically like reading API docs. It's this weird DSL to read and understand.

The best experience would be making the wizard the first experience that users saw and then make it easy to opt--in to a power-mode experience that felt less api-docs like. I suspect most people will stick with the wizard.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Inizia confrontando le esperienze di Copilot CLI e Claude Code descritte nell’issue, prestando particolare attenzione all’onboarding di marketplace e plugin. Definisci cosa dovrebbero includere il wizard guidato e il flusso opzionale in power-mode, quindi verifica che gli utenti possano scoprire, aggiungere e comprendere marketplace e plugin senza dover fare affidamento su documentazione in stile API.

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