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.

review/SKILL.md

@@ -409,6 +409,51 @@ If no documentation files exist, skip this step silently.
 
 ---
 
+## Step 5.7: Codex second opinion (optional)
+
+After completing the review, 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:
+
+```
+Review complete. Want an independent second opinion from Codex (OpenAI)?
+
+A) Run Codex code review — independent diff review with pass/fail gate
+B) Run Codex adversarial challenge — try to find ways this code will fail in production
+C) Both — review first, then adversarial challenge
+D) Skip — no Codex review needed
+```
+
+If the user chooses A, B, or C:
+
+**For code review (A or C):** Run `codex review --base <base>` with a 5-minute timeout.
+Present the full output verbatim under a `CODEX SAYS (code review):` header.
+Check the output for `[P1]` markers — if found, note `GATE: FAIL`, otherwise `GATE: PASS`.
+After presenting, compare Codex's findings with your own review findings from Steps 4-5
+and output a CROSS-MODEL ANALYSIS showing what both found, what only Codex found,
+and what only Claude found.
+
+**For adversarial challenge (B or C):** Run:
+```bash
+codex exec "Review the changes on this branch against the base branch. Run git diff origin/<base> to see the diff. Your job is to find ways this code will fail in production. Think like an attacker and a chaos engineer. Find edge cases, race conditions, security holes, failure modes. Be adversarial." -s read-only
+```
+Present the full output verbatim under a `CODEX SAYS (adversarial challenge):` header.
+
+Persist the Codex review result to the review log:
+```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
+```
+
+If Codex is not available, skip this step silently.
+
+---
+
 ## Important Rules
 
 - **Read the FULL diff before commenting.** Do not flag issues already addressed in the diff.

review/SKILL.md.tmpl

@@ -230,6 +230,51 @@ If no documentation files exist, skip this step silently.
 
 ---
 
+## Step 5.7: Codex second opinion (optional)
+
+After completing the review, 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:
+
+```
+Review complete. Want an independent second opinion from Codex (OpenAI)?
+
+A) Run Codex code review — independent diff review with pass/fail gate
+B) Run Codex adversarial challenge — try to find ways this code will fail in production
+C) Both — review first, then adversarial challenge
+D) Skip — no Codex review needed
+```
+
+If the user chooses A, B, or C:
+
+**For code review (A or C):** Run `codex review --base <base>` with a 5-minute timeout.
+Present the full output verbatim under a `CODEX SAYS (code review):` header.
+Check the output for `[P1]` markers — if found, note `GATE: FAIL`, otherwise `GATE: PASS`.
+After presenting, compare Codex's findings with your own review findings from Steps 4-5
+and output a CROSS-MODEL ANALYSIS showing what both found, what only Codex found,
+and what only Claude found.
+
+**For adversarial challenge (B or C):** Run:
+```bash
+codex exec "Review the changes on this branch against the base branch. Run git diff origin/<base> to see the diff. Your job is to find ways this code will fail in production. Think like an attacker and a chaos engineer. Find edge cases, race conditions, security holes, failure modes. Be adversarial." -s read-only
+```
+Present the full output verbatim under a `CODEX SAYS (adversarial challenge):` header.
+
+Persist the Codex review result to the review log:
+```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
+```
+
+If Codex is not available, skip this step silently.
+
+---
+
 ## Important Rules
 
 - **Read the FULL diff before commenting.** Do not flag issues already addressed in the diff.