dsh-vigenere: Vigenère Cipher Plugin

In the “everything is a plugin” philosophy proposed by DeepSeek Harness (DSH), the dsh-vigenere plugin provides users with a Vigenère cipher implementation in pure Node.js, requiring no network connection to run. It can be installed with the command `dsh plugin add github:uckkk/dsh-vigenere`, and the tools registered by the plugin are automatically loaded into the session. The plugin supports text encryption and decryption, making it especially suitable for secure operations in offline environments. As an open-source project under the MIT license, users can refer to the source code for detailed usage.