Rewrite docs, fix macOS install, fix Bash 3.2 compat

- README.md: complete rewrite with tinyclaw-style centered header,
  shields.io badges, 3-way comparison table (OpenClaw/nanobot/bashclaw),
  Bash 3.2 timeline (2006-2025), ASCII architecture and message flow
  diagrams, accurate test counts (334 tests, 473 assertions),
  full 24-suite test matrix, 8-platform support table
- README_CN.md: complete rewrite synced with English README
- install.sh: fix macOS "command not found" bug by auto-creating
  .zshrc when it doesn't exist (macOS default shell is zsh since
  Catalina), add _verify_install() post-install check, make
  post-install instructions OS-aware (reference correct shell rc)
- lib/cmd_gateway.sh: replace read -ra (Bash 4.0+) with set --
  (Bash 3.2 compatible) in cron_matches_now()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: CrazyBoyM