Enforce gateway-only AI audit execution

[Pigbibi]

Summary

• route AI audit completion exclusively through AIAuditGateway
• fail closed when the gateway is unavailable, rejects the request, or returns a mismatched/empty provider
• forbid custom completion clients, direct OpenAI/Anthropic HTTP calls, and local Codex subprocess fallback
• keep deterministic crisis/TACO routing and execution controls unchanged

Safety boundary

• AI remains shadow-only and cannot mutate routes, orders, allocations, or broker state
• gateway failures emit stable sanitized outcomes
• no production dependency or deployment change

Validation

• PYTHONPATH=src python3 -m pytest -q — 203 passed
• changed-file Ruff — passed
• compileall — passed
• git diff --check — passed
• independent specification and quality review — passed