Add Homebrew formula for easy installation

[Rasukarusan]

Summary

Publish a Homebrew formula so users can install Shellnium with brew install shellnium.

Motivation

• Current installation requires manual git clone
• Homebrew is the standard package manager on macOS
• Lower installation barrier = more users = more stars

Proposed Implementation

• Create a Homebrew tap: homebrew-shellnium
• Formula installs scripts to $(brew --prefix)/lib/shellnium/
• Add shellnium wrapper script to PATH
• Support brew upgrade shellnium for updates
• Also consider Arch Linux AUR package

Usage After Install

brew tap Rasukarusan/shellnium
brew install shellnium

# In scripts:
source "$(brew --prefix)/lib/shellnium/lib/selenium.sh"

Acceptance Criteria

• brew install works on macOS (Intel + Apple Silicon)
• brew install works on Linux (Homebrew on Linux)
• README updated with Homebrew install option