docs: add Mermaid diagrams for network flow

[reoring]

Summary

• Replace ASCII art in README with 3 Mermaid diagrams:
  • Pod overview — Agent → iptables → BotBox → upstream (flowchart)
  • Request processing — allowlist → header rewrite → TLS → stream (flowchart)
  • iptables rules — NAT redirect + filter chain logic (flowchart)
• Replace ASCII sequence diagram in architecture.md with Mermaid sequence diagram
• Add "How it Works" section to README

Test plan

• Mermaid diagrams render correctly on GitHub

🤖 Generated with Claude Code