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.

No files or tests are named. Start by reviewing the current global skill-loading behavior and the closed PR #191, then determine how platform-specific skills could be selected on demand; done requires an agreed design for scoping these skills without exposing them to all Flutter developers.

Written by the indexing model from the issue text.