Remove Anthropic OAuth mention from README

Anthropic doesn't allow this kind of third-party OAuth usage, and the
underlying code (never wired into any UI) was removed on 2.5.2
(64bf704). Cherry-picked just the doc fix to main.

README.md

@@ -150,7 +150,7 @@ Release DMGs are signed and notarized by Apple, so Confab opens normally on firs
 ### API Keys
 Add your API keys in Settings (⌘,) → General tab:
 - **OpenAI** - Get from [OpenAI Platform](https://platform.openai.com/api-keys)
-- **Anthropic** - Get from [Anthropic Console](https://console.anthropic.com/) or use OAuth
+- **Anthropic** - Get from [Anthropic Console](https://console.anthropic.com/)
 - **OpenRouter** - Get from [OpenRouter Keys](https://openrouter.ai/keys)
 - **Ollama** - Base URL (default: http://localhost:11434)
 - **Google** - API key used for Google Custom Search (web search) and Google embeddings (semantic search) - not a chat provider