🤖 chore: sync skills directory from dart-lang/skills

[github-actions]

🤖 Automated Skills Sync & Version Bump (Dart Powered)

This is an automated pull request to sync the contents of the skills/ directory from dart-lang/skills.

This run used a state-tracked Git hash (.dart_skills_githash) computed directly by our custom Dart synchronization utility (tool/sync_skills.dart).

🛠️ Changes Performed:

• Preserved: All native flutter/* skills in the repository remain unchanged.
• Synced: All new, modified, or deleted files inside dart-lang/skills have been carried over.
• Bumped: Automatically incremented the plugin version field in .claude-plugin/plugin.json to keep integrations updated.

[google-cla]

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