Support platform-specific skills (e.g., iOS specific plugin migration)

# Problem
Loading specialized skills like the iOS specific plugin migration ([#191](https://github.com/flutter/agent-plugins/pull/191)) globally exposes them to all Flutter developers, regardless of whether they are working on iOS-specific tasks. That's why this PR was closed (PR feedback)

# Proposed Solution
Explore scoping niche or platform-specific skills that users can select on demand.
• Reduces Noise: Prevents cluttering the core agent's context for general Flutter workflows.
• Better Targeting: Ensures specialized tools are only loaded when explicitly needed for platform-specific tasks.

# Alternatives
Host this skill under a personal repo, or as a GitHub comment.

We plan to reach out to popular plugin authors with this skill included for AI assisted migration. Putting it under a personal account feels a bit sussy. GitHub comment can be hard to manage compared to a centralized repo.

Non sono indicati file o test. Inizia esaminando il comportamento attuale del caricamento globale degli skill e la PR chiusa #191, quindi determina come selezionare su richiesta gli skill specifici della piattaforma; il lavoro sarà considerato completato quando sarà disponibile un design concordato per limitare l'ambito di questi skill senza esporli a tutti gli sviluppatori Flutter.

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