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>

index.html

@@ -8,6 +8,7 @@
   <meta property="og:title" content="김성민 — 풀스택 & 게임 개발자">
   <meta property="og:description" content="김성민의 포트폴리오. 풀스택 개발자이자 게임 개발자.">
   <meta property="og:url" content="https://tolelom.xyz">
+  <meta property="og:image" content="https://tolelom.xyz/images/og.png">
   <meta property="og:type" content="website">
   <link rel="icon" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>👨‍💻</text></svg>">
   <link rel="stylesheet" as="style" crossorigin href="https://cdn.jsdelivr.net/gh/orioncactus/pretendard@v1.3.9/dist/web/variable/pretendardvariable-dynamic-subset.min.css">