feat: integrate /codex into /review, /ship, /plan-eng-review + dashboard

/review offers Codex second opinion after completing its own review.
/ship offers Codex review as optional gate before pushing.
/plan-eng-review offers Codex plan critique after scope challenge.
Review Readiness Dashboard shows Codex Review as optional row.

ship/SKILL.md.tmpl

@@ -402,6 +402,47 @@ For each classified comment:
 
 ---
 
+## Step 3.8: Codex second opinion (optional)
+
+Check if the Codex CLI is available:
+
+```bash
+which codex 2>/dev/null && echo "CODEX_AVAILABLE" || echo "CODEX_NOT_AVAILABLE"
+```
+
+If Codex is available, use AskUserQuestion:
+
+```
+Pre-landing review complete. Want an independent Codex (OpenAI) review before shipping?
+
+A) Run Codex code review — independent diff review with pass/fail gate
+B) Run Codex adversarial challenge — try to break this code
+C) Skip — ship without Codex review
+```
+
+If the user chooses A or B:
+
+**For code review (A):** Run `codex review --base <base>` with a 5-minute timeout.
+Present the full output verbatim under a `CODEX SAYS:` header. Check for `[P1]` markers
+to determine pass/fail gate. Persist the result:
+
+```bash
+eval $(~/.claude/skills/gstack/bin/gstack-slug 2>/dev/null)
+BRANCH_SLUG=$(git rev-parse --abbrev-ref HEAD 2>/dev/null | tr '/' '-')
+mkdir -p ~/.gstack/projects/$SLUG
+echo '{"skill":"codex-review","timestamp":"TIMESTAMP","status":"STATUS","gate":"GATE"}' >> ~/.gstack/projects/$SLUG/$BRANCH_SLUG-reviews.jsonl
+```
+
+If GATE is FAIL, use AskUserQuestion: "Codex found critical issues. Ship anyway?"
+If the user says no, stop. If yes, continue to Step 4.
+
+**For adversarial (B):** Run codex exec with the adversarial prompt (see /codex skill).
+Present findings. This is informational — does not block shipping.
+
+If Codex is not available, skip silently. Continue to Step 4.
+
+---
+
 ## Step 4: Version bump (auto-decide)
 
 1. Read the current `VERSION` file (4-digit format: `MAJOR.MINOR.PATCH.MICRO`)