Configure agent-plugins to use relocated dart_skills_lint repo and test skills

Sub-issue of #201.

Once `dart_skills_lint` is published to its new standalone repository:
- Update `agent-plugins` configuration and workflows to reference the relocated `dart_skills_lint` repository/package.
- Test and verify that all skills in `agent-plugins` validate cleanly with the relocated linter.

Locate the agent-plugins configuration and workflow files that reference dart_skills_lint, then review how the current repository or package is selected. Check the relocated linter's published package and run the existing skill validation workflow or tests across all skills. Done means every agent-plugins skill validates cleanly against the standalone repository.

Written by the indexing model from the issue text.