dsh-llm-oauth: Adding Subscription-Based LLM Login to DeepSeek Harness

`dsh-llm-oauth` is a standalone OAuth plugin for DeepSeek Harness (DSH), designed to allow users to access LLMs through existing subscriptions (such as Grok, GitHub Copilot, ChatGPT/Codex, Anthropic, OpenRouter, Kimi for Coding) without needing to maintain API Keys. The plugin does not modify the Harness repository, reuses the `@earendil-works/pi-ai` model directory, and manages credentials via `CredentialStore` to enable automatic token refresh. Users can access the plugin through the command line or the Web UI's `Settings → OAuth / Subscr` interface.