add completeness validator with substantive-failure error (#2307)
Add IncompleteEvidenceError and validate_completeness to lib/ci_evidence.py: a pure set-difference of the required evidence-gate set (T1) against harvested gates (T3) that raises listing every required gate with no evidence. IncompleteEvidenceError is thin and data-carrying (.missing) with its message defined once for CLI reuse. Implements Task T4 of epic vergil-project/.github#140.