misc/claude-code: Add option USE_CURL_USER_AGENT

Problem:
Certain URLs reject requests from AI agents like Claude.
Example is https://bugs.freebsd.org.

Solution:
The USE_CURL_USER_AGENT option will make Claude to look like Curl.

Benerits to the user:
Claude will be able to read such URLs, summarize their content,
or perform other actions with the contents.
This makes Claude more usable when it comes to such protected URLs.

How it works:
Claude's Fetch tool uses the rgq() function in cli.js which returns:
Claude-User (claude-code/2.1.105; +https://support.anthropic.com/)

This UA is explicitly blocked by Anubis bot protection on bugs.freebsd.org
(and elsewhere), returning a ~2KB "Access Denied" page.

With USE_CURL_USER_AGENT=ON the post-extract-USE_CURL_USER_AGENT-on
target patches rgq() to return curl/8.19.0 (version detected from the
installed curl at build time).