feat: /codex skill — multi-AI second opinion (review, challenge, consult)

Three modes: code review with pass/fail gate, adversarial challenge mode,
and conversational consult with session continuity. First multi-AI skill
in gstack, wrapping OpenAI's Codex CLI.

test/helpers/touchfiles.ts

@@ -73,6 +73,9 @@ export const E2E_TOUCHFILES: Record<string, string[]> = {
   // Document-release
   'document-release': ['document-release/**'],
 
+  // Codex
+  'codex-review': ['codex/**'],
+
   // QA bootstrap
   'qa-bootstrap': ['qa/**', 'browse/src/**', 'ship/**'],
 

test/skill-validation.test.ts

@@ -447,6 +447,7 @@ describe('No hardcoded branch names in SKILL templates', () => {
     'document-release/SKILL.md.tmpl',
     'plan-eng-review/SKILL.md.tmpl',
     'plan-design-review/SKILL.md.tmpl',
+    'codex/SKILL.md.tmpl',
   ];
 
   // Patterns that indicate hardcoded 'main' in git commands