fix: show guide for GitHub Copilot in github install command

[doozie-akshay]

Summary

• Removed the TODO comment in github.ts and added a helpful note when user cancels or doesn't select a provider
• When user cancels the provider selection or selects an invalid provider, they now see a message about GitHub Copilot availability with a link to setup docs

Problem

The kilo github install command was hiding the GitHub Copilot provider without any explanation to users, leaving them unaware of this option.

Solution

Added a helpful note that appears when:

1. User cancels the provider selection (presses Ctrl+C)
2. User doesn't select a valid provider from the list

The message informs users about GitHub Copilot availability and provides a link to setup documentation.

[github-actions]

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