gsd-validate-phase

<objective> Audit Nyquist validation coverage for a completed phase. Three states: - (A) VALIDATION.md exists — audit and fill gaps - (B) No VALIDATION.md, SUMMARY.md exists — reconstruct from artifacts - (C) Phase not executed — exit with guidance <context> Phase: $ARGUMENTS — optional, defaults to last completed phase. </context> <process> Execute end-to-end. Preserve all workflow gates. </process>