feat: OG image, sitemap update, Neo-Slasher, design polish

- Add OG image for social sharing (og:image meta tags)
- Update sitemap data: add Hongik, update descriptions/names
- Add Neo-Slasher project (2D roguelike, ExP game dev club)
- Add Unity client mention to One of the Plans
- Footer border-top, responsive hero photo sizing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

css/style.css

@@ -370,10 +370,12 @@ img { max-width: 100%; display: block; }
 
 /* === Footer === */
 .footer {
-  padding: 40px 0;
+  padding: 48px 0;
   text-align: center;
   color: #999;
   font-size: 0.85rem;
+  border-top: 1px solid #eee;
+  margin-top: 16px;
 }
 .footer a { color: #666; }
 .footer a:hover { color: #333; }
@@ -385,8 +387,10 @@ img { max-width: 100%; display: block; }
   .nav-inner { gap: 16px; }
   .nav-link { font-size: 0.8rem; }
   .hero { padding: 80px 0 56px; }
+  .hero-photo { width: 120px; }
   .hero-name { font-size: 2rem; }
   .hero-role { font-size: 1rem; }
+  .hero-tagline { font-size: 0.9rem; }
   .section { padding: 56px 0; }
   .card-grid { grid-template-columns: 1fr; }
   .infra-grid { grid-template-columns: 1fr 1fr; }