feat: install.sh and uninstall.sh for ~/.local/bin support

[flyingrobots]

Summary

Add POSIX-compatible install and uninstall scripts:

• install.sh — supports npm install -g or --prefix ~/.local
• uninstall.sh — clean removal, supports --prefix
• Checks Node >= 22 and npm availability
• Prints PATH instructions for prefix installs
• Supports curl -fsSL <url> | bash pattern

Acceptance Criteria

• ./install.sh works with default npm global
• ./install.sh --prefix ~/.local installs to ~/.local/bin
• ./uninstall.sh cleanly removes
• Scripts are POSIX-compatible (no bash-isms)