deepseek-ai/deepseek-harness ? reverse-engineered prompt

Build me a simple open source AI agent harness called DeepSeek Harness, with a web app I can run locally and use from my browser.

I want it to feel like a plugin based system where almost everything can be extended, so new tools, behaviors, and integrations can be added without changing the core app too much. Make the main experience a clean web UI where I can start the app, chat with an agent, and manage plugins in one place.

Please include a working local development setup, a build command, and a way to run the web interface from the command line. Keep the project organized so it is easy to add more features later, and make sure the code is ready for a TypeScript based codebase. If you need to check current docs or best practices, look them up online first.