Simplify chaitin-cli skill install and improve robustness

[mmfu123]

Summary

• Remove Windows PowerShell installer (install-chaitin-cli.ps1); Windows users are now guided to manually download from GitHub Releases instead of attempting automated installation
• Add robust GitHub release tag resolution with 3-level fallback: API → redirect → HTML scraping, with GITHUB_TOKEN/GH_TOKEN auth support
• Improve install flow with 3-pass strategy: (1) non-sudo user dirs, (2) sudo system dirs, (3) guaranteed fallback to $HOME/.local/bin
• Add "No-Argument Behavior" section to SKILL.md so /chaitin-cli without args greets, checks/installs, and offers next steps
• Prefer cp+chmod over coreutils install for better portability across sandboxed and minimal environments
• Fix set -u compatibility with empty arrays using ${arr[@]+"${arr[@]}"} pattern

Test plan

• Run /chaitin-cli without arguments and verify greeting + install check flow
• Test on macOS/Linux: run bash scripts/install-chaitin-cli.sh and confirm binary installs correctly
• Test on a machine without chaitin-cli: invoke any chaitin-cli command and verify auto-install + retry
• Verify GitHub API rate-limit scenario triggers redirect/HTML fallback for tag resolution
• Confirm Windows environment shows manual download instructions instead of attempting PowerShell install