dsh-math: Adds mathematical computation and symbolic algebra tools to DeepSeek Harness

`dsh-math` is an MIT-licensed scientific computing plugin for DeepSeek Harness (DSH). It registers the `math` tool, enabling agents to directly handle numerical calculations and symbolic algebra within conversations without switching to external tools. This plugin integrates `mathjs` and `Algebrite` engines, supporting expression evaluation, unit conversion, matrix operations, statistics, complex numbers, and big number handling, while also providing symbolic operations such as simplification, expansion, factorization, differentiation, indefinite integration, and polynomial root finding. Users can install it from npm using `dsh plugin --profile web add dsh-math`. Before installation, users must check the source code, dependencies, and license, and pay attention to its runtime permissions and dependency on `@deepseek-ai/dsh-tools`.