elizax/dsh-http-proxy: Adding Domain-Based Routing Proxy Capabilities for DeepSeek Harness

The `elizax/dsh-http-proxy` plugin addresses a problem in DeepSeek Harness (DSH) where model requests are mixed with other requests (such as search), causing slower performance when using a global proxy or restricted network access when connecting directly. It provides fine-grained control without requiring invasive code changes, allowing users to automatically or manually specify that only model domains use the proxy while all other requests connect directly. It supports multiple protocols, hot configuration, and flexible domain filtering, and remains compatible with the DSH built-in proxy. After installation, users can configure it through the UI or environment variables so that only model API requests go through the proxy, thereby improving overall efficiency.