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/SAX2.h

@@ -95,15 +95,16 @@ XMLPUBFUN void
 		xmlSAX2StartDocument		(void *ctx);
 XMLPUBFUN void
 		xmlSAX2EndDocument		(void *ctx);
-XML_DEPRECATED
+#if defined(LIBXML_SAX1_ENABLED) || defined(LIBXML_HTML_ENABLED) || \
+    defined(LIBXML_WRITER_ENABLED) || defined(LIBXML_LEGACY_ENABLED)
 XMLPUBFUN void
 		xmlSAX2StartElement		(void *ctx,
 						 const xmlChar *fullname,
 						 const xmlChar **atts);
-XML_DEPRECATED
 XMLPUBFUN void
 		xmlSAX2EndElement		(void *ctx,
 						 const xmlChar *name);
+#endif /* LIBXML_SAX1_ENABLED or LIBXML_HTML_ENABLED or LIBXML_LEGACY_ENABLED */
 XMLPUBFUN void
 		xmlSAX2StartElementNs		(void *ctx,
 						 const xmlChar *localname,