feat: add Homebrew Cask installation support

[chetankapoor]

Summary

• Add Homebrew Cask definition (Casks/lil-agents.rb) for project-owned tap
• Add official homebrew-cask submission file (homebrew/lil-agents.rb) as reference
• Update README with Homebrew installation instructions

Project-Owned Tap Setup

To enable brew tap ryanstephen/lil-agents, create a new repo named homebrew-lil-agents on the ryanstephen GitHub account and copy Casks/lil-agents.rb into it:

homebrew-lil-agents/
  └── Casks/
      └── lil-agents.rb

Users can then install via:

brew tap ryanstephen/lil-agents
brew install --cask lil-agents

Official homebrew-cask Submission

The homebrew/lil-agents.rb file is ready to submit as a PR to homebrew/homebrew-cask. Once approved, users can install with just:

brew install --cask lil-agents

At that point, the project-owned tap becomes optional and can be removed.

Test plan

• SHA256 verified against v1.2.2 release zip
• Ruby syntax check passed
• Run brew audit --cask once in a proper tap context
• Confirm README renders correctly with new install section