dsh-standalone-chat: Adding Standalone Chat Feature to DeepSeek Harness

DSH Standalone Chat is a lightweight, standalone chat plugin designed specifically for DeepSeek Harness (DSH). It decouples Agent session logic to provide a clean multi-turn conversation experience, with support for streaming output, history management, and model switching. Conversation data is stored independently and free from context interference. Users can install it via a PowerShell command and launch it from the bottom of the sidebar, making it suitable for pure conversational or debugging scenarios.