ci: harden Codex review workflow (#19)

Co-authored-by: Codex <noreply@openai.com>

Author: Pigbibi

.github/workflows/codex_pr_review.yml

@@ -1,7 +1,7 @@
 name: Codex PR Review
 
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, synchronize, reopened]
 
 permissions:
@@ -19,7 +19,9 @@ jobs:
     uses: QuantStrategyLab/AIAuditBridge/.github/workflows/codex_pr_review.yml@main
     with:
       caller_concurrency_key: pr-${{ github.event.pull_request.number || github.run_id }}
-      allow_unconfigured_backend: true
+      allow_unconfigured_backend: false
+      api_fallback_enabled: "false"
+      direct_api_primary_enabled: "false"
     secrets: inherit
     permissions:
       contents: read