Buildkite Agent improvements

Checkout Skip checkout on any agent by setting BUILDKITE_SKIP_CHECKOUT=true — no plugin or workaround required. Previously this meant clearing BUILDKITE_REPO , using a checkout plugin, or (in the Agent Stack for Kubernetes) setting checkout: skip: true . Skip fetching commits you already have with –git-skip-fetch-existing-commits , which skips the git fetch step during checkout when the target commit already exists locally.