feat: add GitHub Copilot CLI hook integration

[gaburn]

Motivation

This looks like a really cool project and I would love to contribute! I primarily use GitHub Copilot CLI. I humbly present this PR to add GitHub Copilot CLI support.

Summary

Adds a /api/hooks/copilot-cli\ endpoint mirroring the existing Claude Code hook support. GitHub Copilot CLI sessions auto-register as citizens with \copilot-\ prefixed agent IDs and \Copilot\ display branding.

Changes

• New endpoint: \POST /api/hooks/copilot-cli\ — receives Copilot CLI lifecycle events
• New handler: \handleCopilotCliHook()\ with same event-to-state mapping as Claude Code
• New doc: \docs/copilot-cli-quickstart.md\ — setup guide for Copilot CLI users
• Updated: \README.md\ — added Copilot CLI section alongside Claude Code
• Updated: \docs/signal-api.md\ — added Copilot CLI integration section

Design Decisions

• Separate handler method (\handleCopilotCliHook) rather than sharing \handleClaudeCodeHook\ — keeps concerns separated and allows independent evolution as the two hook systems diverge
• Agent IDs use \copilot-\ prefix (vs \claude-) so both can coexist in the same world
• Captures Copilot-specific \model\ field in agent metadata
• Supports same keepalive, sub-agent tracking, and ?agent=/?name=\ query param overrides

Testing

• Verified both endpoints respond with 200
• Confirmed agents from both Copilot CLI and Claude Code appear simultaneously in the world
• Build passes cleanly

[felipe-breda-unipe]

Reviewed the PR

[gaburn]

let me know if you have any comments or suggestions. Would love to merge this if you like it.