Add NH(나무) broker integration and fix balance/holds parsing

- finestock/nh/: Nh/NhV 브로커 클래스 추가 (시세/주문/잔고/실시간 WS)
- api_factory.py, path.py: APIProvider.NH/NHV 등록, 도메인/엔드포인트 매핑
- kis.py: get_holds/get_ohlcv_min/get_index_min/get_stock_list 스텁 추가,
  oauth() Content-Type 헤더 수정
- get_balance()의 실전 디버깅으로 드러난 버그 수정:
  - Hold.total(매입금액)이 존재하지 않는 byn_amt 필드를 참조해 항상 0이던 것을
    eal_amt - eal_pls_amt로 계산하도록 수정
  - rsp_cd를 "00000" 단일 값으로만 성공 판정해 정상 응답('00218' 연속조회 중,
    '00166' 마지막 페이지 등)을 실패로 오판하던 것을 Output_0 존재 여부로 판정
  - 응답 헤더의 cts/cts_flag로 연속조회를 재귀 처리해 10건 넘는 보유종목도
    전부 합쳐서 반환하도록 구현
- doc/, tests/, example_*.py, setup.py, requirements.txt, CLAUDE.md 등 추가
- README.md에 .env 환경변수 설정 가이드 추가

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01225Lu4Fc2UpMz6QixEcNT8

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