Migrate from dart_skills_lint to skills_lint (#9974)

* Migrate from dart_skills_lint to skills_lint

- Update tool/pubspec.yaml dependency from dart_skills_lint to skills_lint
- Rename tool/dart_skills_lint.yaml to tool/skills_lint.yaml and update root key
- Rename .agents/skills/dart_skills_lint_ignore.json to skills_lint_ignore.json
- Update validate_skills_test.dart imports and config file path
- Update validate_skills_hook.sh to use flutter pub run skills_lint

* Update .agents/scripts/validate_skills_hook.sh

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

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Co-authored-by: Reid Baker <1063596+reidbaker@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>