Create Codex AHBG calibration workspace

Author: erinepshovel-code

ahbg/codex/README.md

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+# Codex calibration workspace
+
+Working directory: `stack/ahbg/codex/`
+
+Read `../CALIBRATION.md` first.
+
+Build an independent complete pair here:
+
+```text
+codex/
+├── a0/
+└── ahbg/
+```
+
+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.
+
+Before building, resolve and record exact source commits and applicable skill-lib instructions in `BUILD_MANIFEST.json`.
+
+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`.
+
+## hmmm
+
+Implementation choices are local to this workspace until calibration distinguishes them.