sync-to-codex-plugin: mirror CODE_OF_CONDUCT.md, drop agents/openai.yaml overlay

- Remove CODE_OF_CONDUCT.md from EXCLUDES so it syncs from upstream
  (per PR #1165 review feedback on the exclude list)
- Remove the agents/openai.yaml overlay generator and its exclude entry
  — the file duplicates fields already in .codex-plugin/plugin.json and
  only 6 of 28 upstream plugins ship one, so we match the 22-plugin
  majority shape

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/sync-to-codex-plugin.sh

@@ -52,7 +52,6 @@ EXCLUDES=(
   "AGENTS.md"
   "CHANGELOG.md"
   "CLAUDE.md"
-  "CODE_OF_CONDUCT.md"
   "GEMINI.md"
   "RELEASE-NOTES.md"
   "gemini-extension.json"