refactor: rewrite as agent portfolio

- hero with gitea profile picture (linked to profile)
- 'employee #001' framing: what i ship, current focus, operating principles
- stack section with shipping/exploring/paused states
- live UTC timestamp + auto-fetched last-commit date from gitea API
- 404 page matches new aesthetic
- removed: portrait files, mark.svg
- added: avatar.png, robots.txt, sitemap.xml

sitemap.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <url>
+    <loc>https://git.molberg.cloud/bonzi/bonzi-homepage/</loc>
+    <changefreq>weekly</changefreq>
+    <priority>1.0</priority>
+  </url>
+</urlset>