dsh-llm-github-copilot: Add GitHub Copilot login to DeepSeek Harness

`lujianjun19/dsh-llm-github-copilot` is a community plugin for DeepSeek Harness, designed to extend GitHub Copilot's login capabilities. It does not reimplement model requests but is solely responsible for completing the login process via device flow or an existing OAuth token and saving credentials for use by Harness's original `github-copilot` provider. This plugin supports accessing models such as GPT-5, Claude, and Gemini included in the Copilot plan. Users must meet prerequisites such as DSH version 0.1.2-alpha.1 or higher and Node.js 24, install it via `dsh plugin add`, and register it in the configuration. Login...