Reject malformed closing fences

Authored-By: Codex (GPT-5) <noreply@openai.com>

README.md

@@ -74,7 +74,8 @@ The check validates four things for every non-break-glass Agent PR:
    `https://agenthub.fritzlab.net/bug-<id>` URL. Every PR has a separate
    `## Attribution` section containing the canonical `Authored-By`
    product/model watermark. Agent-authored PR bodies cannot contain HTML
-   comment delimiters, so required provenance cannot be hidden from readers.
+   comment or fenced code delimiters, so required provenance cannot be hidden
+   from readers.
 
 4. **Commit attribution** — every commit in `base-sha..head-sha`, including
    commits on `chore/` branches, ends with

check.sh

@@ -29,6 +29,10 @@ if printf '%s\n' "${BODY}" | grep -Fq '<!--' ||
   echo "FAIL[check-naming]: PR body must not contain HTML comment delimiters"
   FAILED=1
 fi
+if printf '%s\n' "${BODY}" | grep -Eq '^[[:blank:]]*(```|~~~)'; then
+  echo "FAIL[check-naming]: PR body must not contain fenced code delimiters"
+  FAILED=1
+fi
 
 # ---- branch form ----
 # <role>/bug-<id>/<kebab>

tests/run

@@ -162,9 +162,12 @@ check_contract "fences inside comments do not expose provenance" \
 check_contract "escaped backticks do not hide a comment opener" \
   $'\\`<!--\\`\n## Tracking\n- Fixes bug-x7k2m9 — https://agenthub.fritzlab.net/bug-x7k2m9\n\n## Attribution\n- Authored-By: Codex (GPT-5) <noreply@openai.com>\n-->' \
   "$BASE" "$GOOD_HEAD" "PR body must not contain HTML comment delimiters"
-check_contract_pass "fenced code without comments remains valid" \
+check_contract "fenced provenance does not satisfy the visible contract" \
+  $'```\n```oops\n## Tracking\n- Fixes bug-x7k2m9 — https://agenthub.fritzlab.net/bug-x7k2m9\n\n## Attribution\n- Authored-By: Codex (GPT-5) <noreply@openai.com>' \
+  "$BASE" "$GOOD_HEAD" "PR body must not contain fenced code delimiters"
+check_contract "fenced code delimiters are rejected" \
   $'```text\nvisible example\n```\n\n## Tracking\n- Fixes bug-x7k2m9 — https://agenthub.fritzlab.net/bug-x7k2m9\n\n## Attribution\n- Authored-By: Codex (GPT-5) <noreply@openai.com>' \
-  "$BASE" "$GOOD_HEAD"
+  "$BASE" "$GOOD_HEAD" "PR body must not contain fenced code delimiters"
 
 git -C "${FIXTURES}" commit --allow-empty -q -m "unwatermarked change"
 BAD_HEAD=$(git -C "${FIXTURES}" rev-parse HEAD)