Document CI automation for Build Validation Scripts

A Build Validation Scripts best practice is to automate these to run in CI to proactively detect cache regressions. It would be good to document the approach as well as the best practices. The doc should include, but is not limited to:
- Which experiments to automate
- The cadence on which they should automate them
- How to do so (via `--fail-if-not-fully-cacheable`)
- Pointing users towards our GitHub Actions integration

Review the repository's Build Validation Scripts documentation and GitHub Actions integration first. Identify the experiments and automation cadence that should be covered, explain use of --fail-if-not-fully-cacheable, and link users to the GitHub Actions integration. Done means the documented approach covers these points and gives users enough guidance to automate cache-regression checks.

Written by the indexing model from the issue text.