Add sunghyun.is-a.dev subdomain

Points sunghyun.is-a.dev to Vercel (A record) and adds the Vercel
domain ownership verification TXT record.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

domains/_vercel.sunghyun.json

@@ -0,0 +1,9 @@
+{
+  "owner": {
+    "username": "sunghyun-song",
+    "email": "songshyun1@naver.com"
+  },
+  "records": {
+    "TXT": "vc-domain-verify=sunghyun.is-a.dev,217981bfdd78177bd5f1"
+  }
+}

domains/sunghyun.json

@@ -0,0 +1,11 @@
+{
+  "owner": {
+    "username": "sunghyun-song",
+    "email": "songshyun1@naver.com"
+  },
+  "records": {
+    "A": [
+      "216.198.79.1"
+    ]
+  }
+}