GitHub free plan caps macOS jobs at 5; dedupe same-SHA runs

When fanning out many workflow_dispatch builds on separate refs for a public repo on the GitHub Free plan, only about 5 macOS jobs run concurrently and the rest sit 'queued'. Check gh run list --json headBranch,status,headSha for runs of the identical SHA on two refs (e.g. a ci/* ref and the main branch after a fast-forward) and cancel the newer duplicate. Also give each superseding build its own ref (ci/stack-N): re-pushing the same ref triggers concurrency cancel-in-progress and throws away the older run's progress.