Add first class support for Github Copilot CLI

[gtshepard]

Problem

Github Copilot CLI is not a first class citizen in gitagent.

[GitHub Copilot CLI] is a direct alternative to Claude Code as a TUI-based agentic coding tool.

This creates adoption friction for teams that standardize on Copilot CLI and want parity with the existing Claude-oriented experience.

It is currently unclear how to load and use gitagent effectively in Copilot CLI for equivalent functionality

For example, Claude can be invoked directly with:

gitagent run -r https://<mygit>.com/<org>/<repo> -a claude

Solution

Add first-class support for GitHub Copilot CLI with a documented, minimal-configuration workflow equivalent to Claude Code.

For example, support a direct run path such as:

gitagent run -r "https://<mygit>.com/<org>/<repo>" -a copilot

Optionally, add explicit setup/bootstrap support:

gitagent init --assistant copilot

This should clearly define how gitagent is loaded and used within Copilot CLI, so teams can use either TUI-based agentic coding tool with comparable setup effort.