Codex plugin add misses the root plugin.json the list already found
The marketplace entry said 4.5.6. The root plugin.json said 1.2.3. The file under .codex-plugin/ said 9.9.9. codex plugin add reported 9.9.9. It would not have reported any of those strings if the third file had been absent. On that package the same command exited 1, printed missing plugin.json, and did not name the path it had failed to open.
codex plugin list had already printed the source directory. The only manifest in that directory was plugin.json at the plugin root. The file add required was .codex-plugin/plugin.json. Those are not two prints of one file. List can resolve a portable package, and add can still refuse it, before any version comparison runs.