Enrich README with architecture, examples, and full usage docs

[sng2c]

변경사항\n\n- 아키텍처 다이어그램 추가 (두 노드 간 메시지 흐름)\n- 설치 방법 (바이너리 다운로드 / 소스 빌드, darwin/arm64 포함)\n- 모드 설명 (Command 모드, Stdin/Stdout 모드, Pipe 모드)\n- 사용 예시 (채팅, 원격 명령 실행, 파일 전송)\n- 보안 설명 (RSA + AES 암호화)\n- 환경변수 표

---

Note

Low Risk
Documentation-only changes; no runtime behavior or security logic is modified.

Overview
Expands README.md from a brief description into full user documentation: adds an architecture diagram and step-by-step flow, installation instructions (binaries + build-from-source), and a complete flags table (including -blocksize and short options).

Documents operating modes (command vs stdin/stdout vs -pipe), provides practical examples (chat, remote shell, file transfer, custom Redis URL), and adds security/encryption and environment-variable reference sections.

^{Written by Cursor Bugbot for commit 76a09d5. This will update automatically on new commits. Configure here.}