Docs: verify the GitHub Action quickstart in a clean repository

[ChelseaKR]

Goal

Verify the README GitHub Action example in a clean repository and improve any
step that prevents a first-time user from getting annotations.

Test

Create a temporary repository or branch containing only:

• a synthetic TODS sample,
• its approved companion GTFS fixture, and
• the documented workflow.

Do not commit a real agency feed, roster, employee identifier, or vehicle
identifier.

Done when

• The workflow runs against the current supported Action reference.
• A valid fixture passes.
• An intentionally invalid synthetic fixture produces the expected
  annotation and nonzero result.
• The README names every required path and permission.
• Any documentation correction is submitted as a focused pull request.

This issue is documentation and integration testing; it does not require a new
validation rule.