Remove Custom base URL section from Claude Code hook page

Co-Authored-By: Matt Dailey <matt@ref.tools>

Author: devin-ai-integration[bot]

plans/workflows/claude-code-hook.mdx

@@ -31,14 +31,6 @@ export REF_API_KEY=your-api-key
 
 You can find your API key at [ref.tools/keys](https://ref.tools/keys).
 
-### Custom base URL
-
-If you need to point to a different API endpoint (e.g. for staging), set the `REF_BASE_URL` environment variable:
-
-```
-export REF_BASE_URL=https://your-custom-endpoint.example.com
-```
-
 ## How it works
 
 1. You enter `/plan` mode in Claude Code as usual