dsh-env-manager: Environment Variable Management

`dsh-env-manager` is a local Node.js plugin for DeepSeek Harness (DSH), designed to address the pain points of environment variable management. It automatically validates the consistency between code references and `.env` files, identifies missing or unused variables, and generates a sanitized `.env.example` template. The tool does not rely on network access, supports skipping directory scans such as `node_modules`, and quickly manages configuration through the `env_check` and `env_example` commands. It is recommended to review the source code before installation to ensure security.