feat: add GitHub Copilot agent support with ACP

[ereslibre]

Adds GitHub Copilot as a new AI agent in Rover, enabling users to run tasks with Copilot through the Agent Client Protocol (ACP).

Changes

• Added CopilotAgent class in the agent package with credential copying, MCP configuration, and tool argument support
• Added CopilotAI class in the CLI package implementing the full AIAgentTool interface (task expansion, commit messages, merge conflict resolution, GitHub input extraction)
• Extended the ACP runner to support Copilot with the copilot --acp command
• Added Copilot model configurations (gpt-4o, claude-3.5-sonnet, o1)
• Added agent parser support for the copilot identifier
• Updated the AI_AGENT enum to include Copilot

Notes

Copilot uses ACP mode for execution. The agent expects credentials in the ~/.copilot directory and supports GITHUB_TOKEN, GH_TOKEN, and COPILOT_* environment variables.

[Angelmmiguel]

LGTM! Just a minor comment about the models