dsh-dotenv: .env Parsing and Serialization

The `dsh-dotenv` plugin for DeepSeek Harness (DSH) aims to address environment variable management by providing localized parsing and serialization of `.env` files. Implemented in pure Node.js and requiring no network services, all its logic runs locally. Users can install it with `dsh plugin add` and enable it via configuration in `package.json`. Because it involves executing third-party code locally, users should review the source code before use to ensure safety and compliance. The plugin is suitable for scenarios that prioritize dependency-free and lightweight configuration management.