Run linter in CI

**Is your feature request related to a problem? Please describe.**
I'd like to be able to find issues with my Workflow files outside of VS Code (e.g. using a CLI tool that I can run in CI along with all my other linting and testing.

**Describe the solution you'd like**
`gh lint ./github` would be nice

**Additional context**
N/A

Start by reviewing how the repository currently validates GitHub Actions workflow files and how its VS Code support represents that validation. Define the CLI entry point and CI integration for the requested ./github path; done means workflow files can be linted outside VS Code as part of CI, with the command and behavior documented.

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