feat(website): add blog with first post "Why This Exists"

- Blog index at /blog/ with post listing
- First post: project origin story, support expectations, SaaS teaser
- Add Blog link to main site nav and footer
- Update sitemap with blog URLs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/website/index.html

@@ -113,6 +113,7 @@
         <a href="#architecture" class="nav-link">Architecture</a>
         <a href="#screenshots" class="nav-link">Screenshots</a>
         <a href="docs.html" class="nav-link">Docs</a>
+        <a href="blog/" class="nav-link">Blog</a>
         <a href="https://github.com/staack/the-other-dude" class="nav-link" rel="noopener">GitHub</a>
         <a href="docs.html#quickstart" class="nav-cta">Get Started</a>
       </div>
@@ -407,6 +408,7 @@
       </div>
       <nav class="footer-links">
         <a href="docs.html">Docs</a>
+        <a href="blog/">Blog</a>
         <a href="#what-it-does">Features</a>
         <a href="docs.html#security">Security</a>
         <a href="docs.html#api">API Reference</a>