The image build + smoke test + integration suite are the heaviest jobs in CI
(~9-11 min) and ran on every PR. Gate them to push-to-main and release: a
broken build surfaces on the main push, while the cheap pre-merge guards
(docker-lint hadolint/shellcheck, uv-lockfile-check) still run on PRs to
catch the common Dockerfile/lockfile breakage. Steps skip on PRs so the job
stays green; the dead PR-only arm64 cache-warm build is removed.
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