gsd-secure-phase

<objective> Verify threat mitigations for a completed phase. Three states: - (A) SECURITY.md exists — audit and verify mitigations - (B) No SECURITY.md, PLAN.md with threat model exists — run 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>