feat(codex): add plugin (#13757)

plugins/codex/README.md

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+# codex Plugin
+
+## Introduction
+
+This `codex` plugin sets up completion for [codex](https://github.com/openai/codex).
+
+To use it, add `codex` to the plugins array of your zshrc file:
+
+```bash
+plugins=(... codex)
+```

plugins/codex/codex.plugin.zsh

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+# COMPLETION FUNCTION
+if (( ! $+commands[codex] )); then
+  return
+fi
+
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `codex`. Otherwise, compinit will have already done that.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_codex" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _codex
+  _comps[codex]=_codex
+fi
+
+codex completion zsh 2> /dev/null >| "$ZSH_CACHE_DIR/completions/_codex" &|