Workflows now run on GitHub-hosted runners unless the repository
variable USE_BLACKSMITH is "true", so forks work without setup. When
enabled, jobs run on Blacksmith runners (32 vCPU for build/test, 2 vCPU
for lightweight jobs) and use useblacksmith/checkout,
useblacksmith/setup-docker-builder, and useblacksmith/build-push-action.
Docker layer caches are keyed per architecture.
Replaces the CI_RUNNER_X64 and CI_RUNNER_ARM64 variables.
context.issue spreads to { owner, repo, number }, but Octokit v9 requires
issue_number. The request hit /repos/.../issues//labels and returned 404,
so no opened issue was ever labeled.