Reduce max-parallel from 6 to 4 and add aggressive Docker prune with
disk space gate. Runners at 76% disk after prune fail with "Failed to
create temporary directory" errors when overlay2 runs out of space
under parallel DinD load.
If disk usage >= 85% after prune, skip molecule tests for that slot
with a warning instead of letting containers die with cryptic ENOSPC.
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
