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/libexslt/exsltconfig.h

@@ -18,21 +18,21 @@ extern "C" {
  *
  * the version string like "1.2.3"
  */
-#define LIBEXSLT_DOTTED_VERSION "0.8.24"
+#define LIBEXSLT_DOTTED_VERSION "0.8.23"
 
 /**
  * LIBEXSLT_VERSION:
  *
  * the version number: 1.2.3 value is 10203
  */
-#define LIBEXSLT_VERSION 824
+#define LIBEXSLT_VERSION 823
 
 /**
  * LIBEXSLT_VERSION_STRING:
  *
  * the version number string, 1.2.3 value is "10203"
  */
-#define LIBEXSLT_VERSION_STRING "824"
+#define LIBEXSLT_VERSION_STRING "823"
 
 /**
  * LIBEXSLT_VERSION_EXTRA: