Add 4 new articles, internal links, llms.txt, sitemap update

- New articles: M&A due diligence automation, contract review automation,
  how to brief an AI consultant, build vs buy AI automation
- Related articles sections added to all 6 existing articles
- Blog index updated to list all 10 articles
- Sitemap updated with all 10 article URLs
- llms.txt created for AI search engine visibility
- case-studies/index.php: fix title, CSS path, logo path, JS path

blog/articles/what-is-an-ai-agent-professional-services.php

@@ -82,6 +82,15 @@ include($_SERVER['DOCUMENT_ROOT'] . '/includes/nav.php');
             <p>If you have a workflow that currently requires a person to gather information, make sense of it, and take a defined action — there is a good chance an AI agent can handle most of it.</p>
 
         </div>
+
+            <aside class="related-articles">
+                <h2>Related Articles</h2>
+                <ul>
+                    <li><a href="/blog/articles/due-diligence-automation-law-firms">How Law Firms Can Automate Due Diligence Document Review</a></li>
+                    <li><a href="/blog/articles/research-automation-management-consultancy">Research Automation for Management Consultancies</a></li>
+                    <li><a href="/blog/articles/document-extraction-pdf-to-database">Document Extraction: From Unstructured PDF to Structured Database</a></li>
+                </ul>
+            </aside>
         <footer class="article-footer">
             <p>Written by <strong>UK AI Automation</strong> — <a href="/quote">Get a Quote</a></p>
         </footer>