dsh-llm-xai-oauth: Using a SuperGrok / X Premium Subscription in DeepSeek Harness

The DeepSeek Harness plugin integrates SuperGrok/X Premium subscriptions into DSH by reusing local tokens, supporting the Grok 4.x model series. Installation requires Node.js ≥ 22.19, can be completed via `dsh plugin add`, and includes OAuth device-code login. The plugin supports automatic token refresh, but a daemon or scheduled task is required to keep it running. To use it, simply specify `provider xai` and `model grok-4.7` in the configuration to invoke it.