Merge pull request #4 from gudo7208/refactor/code-quality-optimization

fix: address code review issues from superpowers review

refactor: comprehensive code quality and security optimization

docs: add proxy/VPN configuration guide (fixes #3)

feat: add 3-tier fallback strategy for ArXiv weekend resilience

ci: move cron to off-peak UTC 23:51 to avoid scheduler delays

feat: initialize intel-briefing-engine