docs: correct codex adversary advice — strongest model, stdin shape

gpt-5.5 reference was stale (user-level addendum already says
gpt-5.6-sol), and an invocation that neither pipes stdin nor closes it
hangs codex exec forever — both bit this session.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: grrowl

CLAUDE.md

@@ -83,6 +83,9 @@ Each is load-bearing and guarded by an ADR — read it before you touch the area
 - **One decision → one ADR.** Conventional commits (`!` = breaking); end with the
   Co-Authored-By trailer. Commit/push only when asked; `main` is the release
   branch.
-- **Run plans and risky changes past codex (`gpt-5.5`) as an adversary** before
-  committing — it has caught real bugs here (the cursor-fetch race, the
-  scroll-back `DuplicateKeySyncError`).
+- **Run plans and risky changes past codex (`gpt-5.6-sol`) as an adversary**
+  before committing — it has caught real bugs here (the cursor-fetch race, the
+  scroll-back `DuplicateKeySyncError`). Use the strongest available model; a
+  weaker adversary is a weaker review. Pipe input in (`git diff | codex exec
+  … -`) or close stdin (`codex exec "…" < /dev/null`) — an open stdin hangs
+  codex forever.