dsh-is-square: Perfect Square Detection Plugin

`dsh-is-square` is a pure Node.js plugin for DeepSeek Harness (DSH) that locally checks whether a number is a perfect square. It does not rely on the network, requires core libraries such as `@deepseek-ai/cordis`, and its interface definitions should be understood from the source code. It is suitable for instant math computation tasks without internet access, is maintained by developer istone, and is licensed under the MIT License.