session/delete archives instead of deleting #537

CodexAcpClient.deleteSession calls thread/archive, so ACP clients can't permanently delete a session, and ACP has no way to see or restore archived sessions. Codex's app-server has thread/delete. Could session/delete call thread/delete? If archiving is intended, could it be a separate operation instead? I think it would be more clear for delete to equal delete within codex, especially if you want to support real chat deletion without needing to use the desktop app/CLI.