DeepSeek Harness Plugin: dsh-provider-login

The DeepSeek Harness plugin `dsh-provider-login` resolves the missing subscription login entry point in the framework. By mounting an authorization registry and registering commands, it enables users to perform OAuth authorization in a browser and use their ChatGPT Plus/Pro or Claude Pro/Max subscriptions instead of relying directly on API keys. Key features include the `/login` and `/logout` commands and automatic credential refresh. A process restart is required after installation, and note that Anthropic billing will be deducted from additional usage.