feat: add API boundary runner, relocate fixtures, and configure CI check

[reidbaker-agent]

Introduces an automated API boundary verification test runner to programmatically validate the linter's public contract under external packaging constraints (preventing any future regressions where unexported classes or parameters slip into the public interface).

• API Boundary Code Runner: Added a new package at example/api_boundary_runner which imports strictly package:dart_skills_lint/dart_skills_lint.dart (avoiding internal imports) and executes verification logic.
• Fixture Relocation: Moved all reference fixtures to example/skills/valid and example/skills/invalid, updating all references across READMEs, tests, and configurations.
• Workflow Automation: Configured .github/workflows/dart_skills_lint_workflow.yaml to build, analyze, format, and run the example code runner across GHA matrix OS platforms.
• Upgraded downstream validation documentation: Refined [check-downstream-consumers/SKILL.md] to mandate diagnostic API migration checks during PR reviews.

[gemini-code-assist]

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