[HIGH-6] RMF gates are advisory-only — incomplete packages can export

## Problem

The RMF workflow gates at `Implement→Assess` and `Assess→Authorize` transitions are advisory warnings only. No hard block prevents an incomplete or non-compliant package from being exported or submitted. A user can acknowledge the warning and proceed, meaning the gate provides no actual enforcement guarantee for ATO package integrity.

## Evidence / Location

- RMF workflow engine: `Implement→Assess` transition gate
- RMF workflow engine: `Assess→Authorize` transition gate
- Export/submission path: no final completeness check

## Fix

1. Promote gate failures from advisory warnings to hard blocks: incomplete prerequisite conditions must prevent the transition, not merely warn.
2. Define a formal completeness checklist per gate (e.g., all controls implemented before Assess; all findings dispositioned before Authorize).
3. Allow an explicit override only with an elevated role (e.g., `RMF_OVERRIDE`) and log the override with actor identity and justification.

## Source Reviewers

Vision — HIGH

---
> ⚠️ **Caveat:** All findings are static read-reviews; build/tests were not run. Reproduce before treating fix as confirmed.
> 📋 **Note:** War Machine's ATO/RMF findings are still pending and will be folded in as a second batch.