Add Codex repo-specific tracking structure

Codex/README.md

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+# Codex
+
+Codex is the repo-specific execution layer for active work.
+
+Purpose:
+- keep `zlh-grind` root focused on platform-wide context
+- track API, Portal, and Agent work separately
+- make it easier to remove completed items instead of letting them linger in one shared backlog
+
+Use this structure:
+- `Codex/API/*` for `jester/zpack-api`
+- `Codex/Portal/*` for `jester/zpack-portal`
+- `Codex/Agent/*` for `jester/zlh-agent`
+
+File roles:
+- `README.md` — front door for that repo track
+- `CURRENT_STATE.md` — what is actually implemented now
+- `OPEN_ITEMS.md` — only unfinished work
+- `DECISIONS.md` — important settled choices / do-not-re-litigate notes
+
+Operational rules:
+- remove completed items from `OPEN_ITEMS.md` promptly
+- move durable context into `CURRENT_STATE.md` or `DECISIONS.md`
+- keep root `OPEN_THREADS.md` for cross-repo / platform items only
+- if `Codex/*` conflicts with older historical notes elsewhere in `zlh-grind`, prefer the newer Codex entry unless there is stronger evidence in the source repo