build: make npm run build self-sufficient

The ts-cleaner step in the build script scans dist/ and crashes with
ENOENT if the directory doesn't exist (e.g. on a fresh clone or after
git clean). Previously README told users to 'mkdir -p dist' first,
but Dockerfile and CI didn't necessarily follow that. Prepend a small
node one-liner that mkdir's dist recursively before ts-cleaner runs,
and drop the now-redundant manual step from README.

README.md

@@ -85,8 +85,7 @@ npm install
 # 개발 (ts-node)
 npm run dev
 
-# 빌드 후 실행 (ts-cleaner가 dist/를 스캔하므로 먼저 만들어야 함)
-mkdir -p dist
+# 빌드 후 실행 (build 스크립트가 dist 디렉터리를 자동 생성)
 npm run build
 npm start