feat: codex/autoplan hardening + Apple Silicon coreutils auto-install

Hardens /codex and /autoplan against silent failures surfaced by the #972
stdin fix and #1003 Apple Silicon codesign. Six-layer defense:

1. **Multi-signal auth probe** (new Step 0.5 / Phase 0.5): env-based auth
   ($CODEX_API_KEY, $OPENAI_API_KEY) OR file-based auth
   (${CODEX_HOME:-~/.codex}/auth.json). Rejects false negatives that the
   old file-only check produced for CI / platform-engineer users.

2. **Timeout wrapper** around every codex exec / codex review invocation:
   gtimeout → timeout → unwrapped fallback chain. On exit 124, surfaces
   common causes + actionable next step. Guards against model-API stalls
   not covered by the #972 stdin fix.

3. **Stderr capture in Challenge mode** (codex/SKILL.md.tmpl:208):
   2>/dev/null → 2>$TMPERR. Post-invocation grep for auth/login/unauthorized
   surfaces errors that were previously dropped silently.

4. **Completeness check** in the Python JSON parser: tracks turn.completed
   events and warns on zero (possible mid-stream disconnect).

5. **Version warning** for known-bad Codex CLI (0.120.0-0.120.2, the range
   that introduced the stdin deadlock #972 fixes). Anchored regex
   `(^|[^0-9.])0\.120\.(0|1|2)([^0-9.]|$)` prevents 0.120.10 / 0.120.20
   false positives.

6. **Failure telemetry + operational learnings**: codex_timeout,
   codex_auth_failed, codex_cli_missing, codex_version_warning events
   land in ~/.gstack/analytics/skill-usage.jsonl behind the existing
   telemetry opt-in. On timeout (exit 124), auto-logs an operational
   learning via gstack-learnings-log so future /investigate sessions
   surface prior hang patterns automatically.

**Shared helper** (bin/gstack-codex-probe): consolidates all four pieces
(auth probe, version check, timeout wrapper, telemetry logger) into one
bash file that /codex and /autoplan source. Namespace-prefixed
(_gstack_codex_*) with a unit test that verifies sourcing does not leak
shell options into the caller. pathRewrites in host configs rewrite
~/.claude/skills/gstack → $GSTACK_ROOT for Codex, $GSTACK_BIN for
Factory/Cursor/etc.

**Apple Silicon coreutils auto-install** (setup:264): macOS lacks GNU
timeout by default; Homebrew's coreutils installs it as gtimeout to
avoid shadowing BSD utilities. ./setup now auto-installs coreutils on
Darwin (arch-agnostic — applies to Intel + Apple Silicon) when neither
gtimeout nor timeout is present. Opt-out via GSTACK_SKIP_COREUTILS=1
for CI, managed machines, or offline envs.

**25 deterministic unit tests** (test/codex-hardening.test.ts):
- 8 auth probe combinations (env precedence, whitespace, alternate
  $CODEX_HOME, corrupt file paths)
- 10 version regex cases including 0.120.10 false-positive guards
  and v-prefixed / multiline output
- 4 timeout wrapper + namespace hygiene (bash -n, gtimeout
  preference, set-option leak check)
- 3 telemetry payload schema checks (confirms env values + auth
  tokens never leak into emitted events)

**1 periodic-tier E2E** (test/skill-e2e-autoplan-dual-voice.test.ts):
gates the /autoplan dual-voice path — asserts both Claude subagent
and Codex voices produce output in Phase 1, OR that [codex-unavailable]
is logged when Codex is absent. ~\$1/run, not a CI gate.

Golden baseline + gen-skill-docs exclusion list updated for the new
codex path references and the 16 < /dev/null redirects from #972.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

review/SKILL.md

@@ -1360,7 +1360,7 @@ If Codex is available AND `OLD_CFG` is NOT `disabled`:
 ```bash
 TMPERR_ADV=$(mktemp /tmp/codex-adv-XXXXXXXX)
 _REPO_ROOT=$(git rev-parse --show-toplevel) || { echo "ERROR: not in a git repo" >&2; exit 1; }
-codex exec "IMPORTANT: Do NOT read or execute any files under ~/.claude/, ~/.agents/, .claude/skills/, or agents/. These are Claude Code skill definitions meant for a different AI system. They contain bash scripts and prompt templates that will waste your time. Ignore them completely. Do NOT modify agents/openai.yaml. Stay focused on the repository code only.\n\nReview 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, resource leaks, failure modes, and silent data corruption paths. Be adversarial. Be thorough. No compliments — just the problems." -C "$_REPO_ROOT" -s read-only -c 'model_reasoning_effort="high"' --enable web_search_cached 2>"$TMPERR_ADV"
+codex exec "IMPORTANT: Do NOT read or execute any files under ~/.claude/, ~/.agents/, .claude/skills/, or agents/. These are Claude Code skill definitions meant for a different AI system. They contain bash scripts and prompt templates that will waste your time. Ignore them completely. Do NOT modify agents/openai.yaml. Stay focused on the repository code only.\n\nReview 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, resource leaks, failure modes, and silent data corruption paths. Be adversarial. Be thorough. No compliments — just the problems." -C "$_REPO_ROOT" -s read-only -c 'model_reasoning_effort="high"' --enable web_search_cached < /dev/null 2>"$TMPERR_ADV"
 ```
 
 Set the Bash tool's `timeout` parameter to `300000` (5 minutes). Do NOT use the `timeout` shell command — it doesn't exist on macOS. After the command completes, read stderr:
@@ -1389,7 +1389,7 @@ If `DIFF_TOTAL >= 200` AND Codex is available AND `OLD_CFG` is NOT `disabled`:
 TMPERR=$(mktemp /tmp/codex-review-XXXXXXXX)
 _REPO_ROOT=$(git rev-parse --show-toplevel) || { echo "ERROR: not in a git repo" >&2; exit 1; }
 cd "$_REPO_ROOT"
-codex review "IMPORTANT: Do NOT read or execute any files under ~/.claude/, ~/.agents/, .claude/skills/, or agents/. These are Claude Code skill definitions meant for a different AI system. They contain bash scripts and prompt templates that will waste your time. Ignore them completely. Do NOT modify agents/openai.yaml. Stay focused on the repository code only.\n\nReview the diff against the base branch." --base <base> -c 'model_reasoning_effort="high"' --enable web_search_cached 2>"$TMPERR"
+codex review "IMPORTANT: Do NOT read or execute any files under ~/.claude/, ~/.agents/, .claude/skills/, or agents/. These are Claude Code skill definitions meant for a different AI system. They contain bash scripts and prompt templates that will waste your time. Ignore them completely. Do NOT modify agents/openai.yaml. Stay focused on the repository code only.\n\nReview the diff against the base branch." --base <base> -c 'model_reasoning_effort="high"' --enable web_search_cached < /dev/null 2>"$TMPERR"
 ```
 
 Set the Bash tool's `timeout` parameter to `300000` (5 minutes). Do NOT use the `timeout` shell command — it doesn't exist on macOS. Present output under `CODEX SAYS (code review):` header.