fix(ci): close two silent false negatives in Identity Guard

An audit found the guard could miss commits it exists to catch.

1. SIGPIPE. Three inner checks used `printf ... | grep -q` under
   `set -euo pipefail`. When grep -q matches early it exits, printf dies of
   SIGPIPE, pipefail turns the pipeline non-zero, the `if` goes false, and the
   offending commit is silently not reported — the guard then prints its
   success line. This file already documents the hazard in a comment and had
   fixed it for the two outer pre-filters; the three inner ones were left as
   pipes. Now here-strings, like the rest.

   Demonstrated on a synthetic revert-style commit (an AI trailer quoted near
   the top, then a 200KB body): the pipe form MISSES it, the here-string form
   detects it. A trailer in the conventional last position happens to survive
   the pipe form, because grep must read to the end before matching — which is
   why this never showed up in practice.

2. `chatgpt` was in BAD_NAME and BAD_ANY but not BAD_TRAILER, so a
   Co-authored-by naming ChatGPT with an email outside anthropic.com/openai.com
   was not caught. Confirmed by test, then added.

Verified before pushing this time: the amended script flags an AI-authored
commit, a large-body early trailer, and a ChatGPT trailer, and scans
claude_guide's full 345-commit history with zero false positives — that repo
has 46 commits with Claude or Anthropic in the subject, none of which are
identity or trailer hits.

.github/workflows/identity-guard.yaml

@@ -81,7 +81,7 @@ jobs:
           # 的书），所以只检查身份字段与 Co-authored-by 尾注，绝不扫描自由文本。
           BAD_EMAIL='@([a-z0-9.-]+\.)?(anthropic|openai)\.com$'
           BAD_NAME='^(claude([ ._-]?code)?|codex|chatgpt|copilot|anthropic|openai)(\[bot\])?$'
-          BAD_TRAILER='^[[:space:]]*co-authored-by:.*(claude|anthropic|codex|copilot|openai)'
+          BAD_TRAILER='^[[:space:]]*co-authored-by:.*(claude|anthropic|codex|copilot|openai|chatgpt)'
           # 粗筛：BAD_EMAIL 与 BAD_NAME 命中的行必然含下列词之一，
           # 因此它是二者的超集，用来跳过绝大多数干净提交，不会漏判。
           BAD_ANY='(claude|anthropic|codex|chatgpt|copilot|openai)'
@@ -101,8 +101,8 @@ jobs:
               if [ "$role" = author ]; then name="$an"; mail="$ae"; else name="$cn"; mail="$ce"; fi
               lname="$(printf '%s' "$name" | tr '[:upper:]' '[:lower:]')"
               lmail="$(printf '%s' "$mail" | tr '[:upper:]' '[:lower:]')"
-              if printf '%s' "$lmail" | grep -qE "$BAD_EMAIL" \
-                 || printf '%s' "$lname" | grep -qE "$BAD_NAME"; then
+              if grep -qE "$BAD_EMAIL" <<<"$lmail" \
+                 || grep -qE "$BAD_NAME" <<<"$lname"; then
                 echo "::error::${sha} ${role} identity is an AI assistant: ${name} <${mail}>"
                 failed=1
               fi
@@ -118,7 +118,7 @@ jobs:
             while read -r sha; do
               [ -n "$sha" ] || continue
               body="$(git log -1 --format='%B' "$sha")"
-              if printf '%s' "$body" | grep -qiE "$BAD_TRAILER"; then
+              if grep -qiE "$BAD_TRAILER" <<<"$body"; then
                 echo "::error::${sha} has a Co-authored-by trailer referencing an AI assistant"
                 printf '%s' "$body" | grep -iE "$BAD_TRAILER" | sed 's/^/    /'
                 failed=1