dsh-provider-proxy: DeepSeek Harness Scoped Proxy Middleware

`dsh-provider-proxy` is a middleware plugin for DeepSeek Harness (DSH), designed to address the issue that a single global proxy configuration is overly broad. It allows users to configure HTTP/HTTPS proxies for specific Providers, enabling Provider-level network isolation while other Providers remain directly connected. The plugin is implemented based on `globalThis.fetch`, supports configuration via the UI or YAML, and allows proxy authentication using environment variables. After installation, it must be enabled in settings, and the service must be restarted for the change to take effect. It does not support SOCKS proxies and does not rewrite a Provider's `baseURL`.