Add Static analysis tool written in dart that checks for skill compliance with the open skills specification.

[reidbaker]

Design doc (googler only) go/flutter-dart-skills-lint-publish-plan
Fixes: #59

Review overview:

• .agent/skills/
• • add-dart-lint-validation-rule is a custom skill for working on the linter.
• • The rest were installed with npx. I could use a second opinion about what should be checked in from hosted skills.
• feature_design_docs/ are ai artifacts of unimplemented features. I don't think the contents need review but I am open to opinions about if this kind of thing is something we are ok with checking in.
• knowledge/ is for docs about the structure of the lint codebase. Also ai artifacts. You are welcome to review contents but it is a lower priority.
• lib/src/models
• • These are "just" simple dart models but that means the bar for quality is high. Would appreciate review on on both the structure and documentation/understandability of all of the model objects.
• lib/src
• • This is the bulk of the actual code
• • Would appreciate review on patterns that are bad/wrong and for structural feedback (e.g better method organization)
• tests
• • Most important: Missing classes of tests or areas you think that are easy to regress
• • Patterns that are bad across all the test.
• README.md
• • Any documentation you see that is incorrect
• • Questions you would have as a consumer that are not answered.

Finally any advice you have to make this code more maintainable moving forward. How to integrate it into your cicd etc. I think ci changes should go into a second pr but it may still be valuable to talk about it here.

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.

[reidbaker]

/gemini review