docs: add GitHub Copilot CLI installation instructions

[GeekTrainer]

Purpose

Adds GitHub Copilot CLI installation instructions to the README, following the existing pattern for VS Code setup.

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:

How to Test

• View the rendered README.md on GitHub
• Verify JSON configuration is valid

What to Check

• Markdown renders correctly
• JSON config matches existing patterns in the repo

Other Information

N/A - Documentation-only change