self-hosted-runners

Use when running GitHub Actions on local self-hosted runners instead of GitHub-hosted ones, or when reviewing such a setup. One repository variable (RUNNER_LABELS, a JSON array) switches every job, with cloud as the default; the guard for jobs that need an OS the host lacks; release and secret-bearing jobs that stay on cloud; runner host setup with ephemeral registration and re-minted tokens; concurrency and timeouts; the security rules for running pull-request code on your own machine; operations and the offline-runner fallback; and the agent procedure that writes docs/RUNNER.md.