feat: /codex skill — multi-AI second opinion + proactive suggestions (#197)

* 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.

* 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.

* chore: bump version and changelog (v0.8.0)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: codex skill validation (12 stub tests) + E2E eval test

Stub tests (free tier): verify template content — three modes, gate verdict,
session continuity, cost tracking, cross-model comparison, binary discovery,
error handling, mktemp usage, and integrations into /review, /ship, /plan-eng-review.

E2E test (paid tier): runs /codex review on vulnerable fixture repo via
session-runner, verifies output contains findings and GATE verdict.

* fix: codex auth error message — use codex login, not OPENAI_API_KEY

Codex authenticates via ChatGPT OAuth (codex login), not an env var.

* feat: codex uses high reasoning effort by default

gpt-5.2-codex is the only model available with ChatGPT login.
All commands now use model_reasoning_effort="high" for maximum
depth — the whole point is a thorough second opinion.

* feat: crank codex reasoning to xhigh (maximum)

* feat: per-mode reasoning (high for review/consult, xhigh for challenge) + web search

Review and consult use high reasoning — thorough but not slow.
Challenge (adversarial) uses xhigh — maximum depth for breaking code.
All modes enable web_search_cached so Codex can look up docs/APIs.

* refactor: don't hardcode model — use codex default (always latest)

* feat: JSONL output for codex challenge + consult modes

Use --json flag to parse codex's JSONL events, extracting reasoning
traces ([codex thinking]), tool calls ([codex ran]), and token counts.
This gives richer output than the -o flag alone — you can see what
codex thought through before its answer.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: only persist codex-review log when code review actually ran

Don't write a codex-review entry to reviews.jsonl when only the
adversarial challenge (option B) was selected — there's no gate
verdict to record, and a false entry misleads the Review Readiness
Dashboard into thinking a code review happened.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: add codex plan review option to /plan-eng-review

After scope challenge (Step 0), offer to have Codex independently
review the plan with a brutally honest tech reviewer persona.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: update e2e test for codex skill

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: codex integration bugs — plan content, review persistence, quoting, stderr

- plan-eng-review: Codex now reads the plan file itself instead of inlining
  content as a CLI arg (avoids ARG_MAX for large plans)
- review: add missing echo to persist codex-review results to reviews.jsonl
- codex: consult mode uses $TMPERR (mktemp) instead of hardcoded stderr path
- codex + review: quote $SLUG/$BRANCH_SLUG in review log paths
- codex: scope plan lookup to current project, warn on cross-project fallback

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: add .context/ to .gitignore to prevent session ID leaks

Codex consult mode stores session IDs in .context/codex-session-id.
Without this ignore rule, session IDs could leak into commits.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: proactive skill suggestions + opt-out + trigger phrase tests

- Preamble reads proactive config via gstack-config
- Root SKILL.md.tmpl has lifecycle map (stage → skill suggestion)
- Users can opt out ("stop suggesting") / opt in ("be proactive again")
- Restored trigger phrase validation tests (16 skills × "Use when" check)
- Added missing "Use when" trigger phrases to /debug and /office-hours

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: update changelog for v0.8.0 — add proactive suggestions note

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>