docs: clarify Anthropic-only LLM support

https://claude.ai/code/session_018nuwtRWVYDAxHzQ2r1Rk64

Author: claude

README.md

@@ -530,7 +530,7 @@ See [CLAUDE.md](CLAUDE.md) for repo structure and contributor notes.
 ## Known limitations
 
 - **Single entry point** — only one preview URL or start command per run is supported; multiple entry points are planned.
-- **LLM provider** — tested with Anthropic; other providers may work but aren't officially supported yet.
+- **LLM provider** — only Anthropic is supported currently; support for other providers is planned.
 
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