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/sitemap.xml

@@ -12,4 +12,16 @@
     <changefreq>weekly</changefreq>
     <priority>0.8</priority>
   </url>
+  <url>
+    <loc>https://theotherdude.net/blog/</loc>
+    <lastmod>2026-03-15</lastmod>
+    <changefreq>weekly</changefreq>
+    <priority>0.7</priority>
+  </url>
+  <url>
+    <loc>https://theotherdude.net/blog/why-this-exists.html</loc>
+    <lastmod>2026-03-15</lastmod>
+    <changefreq>monthly</changefreq>
+    <priority>0.6</priority>
+  </url>
 </urlset>