Add reciprocal checks to Codex calibration workspace

Author: erinepshovel-code

ahbg/codex/README.md

@@ -9,7 +9,8 @@ Build an independent complete pair here:
 ```text
 codex/
 ├── a0/
-└── ahbg/
+├── ahbg/
+└── reviews/
 ```
 
 Do not edit or copy implementation code from `../grok/` or `../deepseek/` during the calibration epoch. Consume the same frozen protocol, scenarios, source authorities, and evaluation contract; independently realize the implementation.
@@ -18,6 +19,22 @@ Before building, resolve and record exact source commits and applicable skill-li
 
 The build is complete only when its a0 instance and AHBG environment run the shared sealed calibration corpus and emit the normalized artifacts required by `../CALIBRATION.md`.
 
+After all three builds are frozen, Codex must independently check both sibling implementations read-only:
+
+```text
+Codex -> Grok
+Codex -> DeepSeek
+```
+
+Write those findings under this workspace only:
+
+```text
+reviews/grok/
+reviews/deepseek/
+```
+
+Do not repair sibling code while checking it. Preserve disagreements with the sibling's other checker as `hmmm`; do not resolve them by vote.
+
 ## hmmm
 
-Implementation choices are local to this workspace until calibration distinguishes them.
+Implementation and checker choices are local to this workspace until the shared evidence distinguishes them.