Agent Skills: dart-tooling

Dart static analysis, linting, formatting, and code-generation standards. Use when touching analysis_options.yaml, running build_runner, configuring dart format line length, setting up DCM metrics, or adding pre-commit hooks via lefthook — and whenever a CI job fails on analyze or format steps. (triggers: analysis_options.yaml, build.yaml, build_runner, lefthook.yml, dart format, dart_code_metrics)