dsh-atbash: Atbash Cipher Conversion Tool

dsh-atbash is a lightweight security plugin in the DeepSeek Harness ecosystem that implements the Atbash cipher algorithm. The plugin is implemented in pure Node.js and requires no network dependencies. Developers can install it with the `dsh plugin add` command and must ensure that `@deepseek-ai/cordis` and `@deepseek-ai/dsh-tools` are installed. The plugin runs with the current process permissions and is suitable for basic reverse-alphabet substitution use cases.