Add a skill for linter

[reidbaker]

Author a plain jane skill in the format of https://pub.dev/packages/skills.

I will need some help (and an auth token) to be able to setup this skill using the more complicated contributing documentation for skills in this repo.

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 the Flutter Style Guide recently, and have followed its advice.
• 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.

[gemini-code-assist]

Code Review

This pull request adds documentation for the dart_skills_lint package, including a general README for shipped skills and a specific guide for the dart-skills-lint-validation skill. The feedback identifies a version inconsistency in the setup instructions and recommends adding an assertion to the provided test example to ensure that validation failures are properly reported.

[reidbaker]

After talking to @jwren out of band the plan is to remove the integration with validate-skill and land the skill as is.
I will file an issue with the bugged validate-skill behavior for different directories.