Update actions/checkout action to v3

Editor assessment

`build: update actions/checkout action to v3` is a model minor-bump subject: it names the exact dependency and target version, so the change is fully understandable without a diff. The absent body is justified — a CI action version bump needs no rationale, and the rubric forgives missing bodies on small changes. No footers exist; none are required and none are malformed. Style hygiene is excellent: imperative verb, no trailing period, short subject, no ticket junk, consistent language. Teaching value is inherently minimal because there is nothing to explain, but as a clear, correct, self-sufficient message it satisfies the good bar without qualification.