1. First Principles of CI/CD Automation

Before touching a single YAML key, we need the model that every GH-200 question secretly tests: something happens in a repository, and a machine somewhere reacts to it in a precisely defined way. This phase builds that model — why automation exists at all, how GitHub Actions structures a reaction to an event, and why "reuse" comes in three distinct flavors. Everything in Phases 2 through 6 is a refinement of what you learn here.