feat(agent): scrape linked pages to ground answers in real content

Adds `_fetch_page_content()`, which downloads a URL with a
browser-like User-Agent and uses BeautifulSoup with the lxml
parser to strip scripts, navigation, and other non-content
markup, returning up to 3000 characters of readable text.

The response validator now also compares generated replies
against this scraped text, flagging named entities and claims
that do not appear in the retrieved page content.

Dependencies: adds beautifulsoup4==4.12.3 and pins lxml==5.3.0
for parser compatibility.

venv/lib/python3.11/site-packages/lxml/includes/libxml/xlink.h

@@ -160,20 +160,16 @@ struct _xlinkHandler {
  * detection callbacks.
  */
 
-XML_DEPRECATED
 XMLPUBFUN xlinkNodeDetectFunc
 		xlinkGetDefaultDetect	(void);
-XML_DEPRECATED
 XMLPUBFUN void
 		xlinkSetDefaultDetect	(xlinkNodeDetectFunc func);
 
 /*
  * Routines to set/get the default handlers.
  */
-XML_DEPRECATED
 XMLPUBFUN xlinkHandlerPtr
 		xlinkGetDefaultHandler	(void);
-XML_DEPRECATED
 XMLPUBFUN void
 		xlinkSetDefaultHandler	(xlinkHandlerPtr handler);