refactor: rename stock-lab → stock (graduation)

- git mv stock-lab/ → stock/
- docker-compose.yml: 서비스 키 + container_name + build.context +
  frontend.depends_on + agent-office STOCK_LAB_URL → STOCK_URL
- agent-office/app: config.py, service_proxy.py, agents/stock.py, tests/
  STOCK_LAB_URL → STOCK_URL
- nginx/default.conf: proxy_pass http://stock-lab → http://stock (3 lines)
- CLAUDE.md / README.md / STATUS.md / scripts/ 문구 갱신
- stock/ 내부 자기 참조 갱신

lab 네이밍 정책 (feedback_lab_naming.md) graduation.
API URL / Python import / DB 파일명 변경 없음.

stock/app/holidays.json

@@ -0,0 +1,18 @@
+[
+  "2026-01-01",
+  "2026-01-28",
+  "2026-01-29",
+  "2026-01-30",
+  "2026-03-01",
+  "2026-05-05",
+  "2026-05-25",
+  "2026-06-06",
+  "2026-08-15",
+  "2026-09-24",
+  "2026-09-25",
+  "2026-09-26",
+  "2026-10-03",
+  "2026-10-09",
+  "2026-12-25",
+  "2026-12-31"
+]