dsh-advanced-provider-settings: DeepSeek Harness Advanced Provider Configuration Plugin

The native interface of DeepSeek Harness (DSH) only supports basic configuration, while many critical parameters—such as retry policies, timeouts, and reasoning settings—remain hidden in YAML files. The `dsh-advanced-provider-settings` plugin makes these hidden settings visible through the WebUI, enabling fine-grained management. It supports third-party endpoint configuration (such as dialect declaration and field spelling corrections), directory-routing model overrides, and global provider settings (including User-Agent, retry policies, and compatibility flags), while using a safe-write mechanism to preserve configuration files exactly as they are. This plugin lowers the barrier to integrating third-party DeepSeek endpoints and improves DSH's debugging and maintenance efficiency.