Add Copilot agent

[devzeebo]

Summary

Add a first-class GitHub Copilot agent (Copilot CLI / coding agent) so harness tests can target Copilot with the same Given/When/Then API.

Context

Today agentRegistry only exports cursor. New agents should plug into the existing registry, image build, and executing_the_agent flow.

Acceptance criteria

• Register a copilot agent in the agent registry
• Provide a Copilot agent Docker image on top of agent-gwt/base:local
• Implement run that mounts only the credentials Copilot needs, runs as the host user, and returns a parsed result compatible with AgentContext
• Support buildAgentImage("copilot") from vitest globalSetup
• Document auth prerequisites and a minimal usage example in the README
• Unit/integration coverage consistent with the Cursor agent patterns

Notes

Prefer src/agents/copilot/ + docker/copilot/ mirroring the Cursor agent.