codex-maintenance.sh evaluated each affected host/package one at a time,
even though those calls are independent. Added scripts/lib/nix-parallel.sh
(run_nix_parallel) and wired it into the host-eval, package-eval, and
dry-run-build loops.
Concurrency defaults to core count capped by available memory (~1GB/job)
rather than plain nproc: empirically, nproc-many concurrent full-flake
evals OOM-killed each other on a 4GB/6-core box, while 3-4 ran clean and
were still ~2x faster than serial. Override via NIX_PARALLEL_JOBS.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
