dsh-hmac: HMAC Message Authentication Code

`uckkk/dsh-hmac` is a pure Node.js local HMAC computation plugin for DeepSeek Harness (DSH). It requires no network connection and supports message authentication code (MAC) computation and local encryption. Users must install it using `dsh plugin add` and enable it in the profile's `package.json`. The plugin is suitable for local encryption verification scenarios, but users should review the source code themselves to ensure security.