Migrate from dart_skills_lint to skills_lint

[reidbaker-agent]

Follows the migration guide in google/skills_lint.dart to migrate DevTools from dart_skills_lint to skills_lint.

Changes

• Update tool/pubspec.yaml dependency to use skills_lint from https://github.com/google/skills_lint.dart.git.
• Rename tool/dart_skills_lint.yaml to tool/skills_lint.yaml and update top-level YAML key to skills_lint:.
• Rename .agents/skills/dart_skills_lint_ignore.json to .agents/skills/skills_lint_ignore.json.
• Update tool/test/validate_skills_test.dart imports and config path.
• Update .agents/scripts/validate_skills_hook.sh to use flutter pub run skills_lint.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[reidbaker-agent]

cc @reidbaker for review