feat: Add ability to run via copilot

[mertse7en]

Add Copilot CLI support

Ralph can now use Copilot CLI as a backend, alongside Amp and Claude Code, for the autonomous agent loop.

Summary

• Adds --tool copilot support to the Ralph runner.
• Invokes Copilot CLI non-interactively with:

  copilot -p "$(cat COPILOT.md)" --allow-all-tools

-Introduces a dedicated COPILOT.md prompt template for Copilot-driven iterations.
Updates the docs so Copilot CLI appears as a first-class supported tool in setup, usage, and key file references.

Changes

• ralph.sh
• Adds --tool copilot validation.
• Updates the invalid-tool error message.
• Adds a Copilot execution branch using -p plus --allow-all-tools.
• COPILOT.md
• New Copilot-specific agent prompt template.
• Matches the Ralph workflow and completion contract.
• README.md
• Adds Copilot CLI prerequisites.
• Adds setup instructions.
• Adds prompt-copy instructions.
• Adds workflow usage.
• Updates key files entry and references.
• AGENTS.md
• Adds Copilot CLI to the overview.
• Updates commands.
• Updates key files.
• Updates agent-loop patterns.