* test(#3833): failing-first suite for the PR mergeability preflight
* chore(#3833): gate every PR compute lane behind a mergeability preflight
* fix(#3833): assert the preflight gate on parsed yaml and guard status-function if
* fix(#3833): run stub-backed cli tests in-process to avoid a spawnsync deadlock
* fix(#3833): fail the preflight open when its script is absent at the base sha
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Co-authored-by: sim <sim@local>
