Add GitHub Copilot CLI as a supported host

[ridermw]

Summary

The GitHub Copilot CLI went GA in February 2026 as a standalone binary (copilot), separate from the old gh copilot extension. It supports skill discovery via ~/.copilot/skills/ and has its own session storage at ~/.copilot/session-state/{id}/.

gstack already supports Claude Code, Codex, Gemini CLI, and Cursor. Copilot CLI fits the same pattern — it uses .agents/skills/ for repo-local discovery and has a global install path at ~/.copilot/skills/gstack.

What would Copilot support look like?

Based on the existing Codex pattern:

• gen-skill-docs.ts: Add 'copilot' host type. Copilot shares .agents/skills/ output with Codex — only the global fallback path differs (~/.copilot/skills/gstack vs ~/.codex/skills/gstack).
• setup: Add --host copilot with auto-detection via command -v copilot. Rewrite Codex output paths at install time via sed (same pattern as Kiro).
• gstack-global-discover.ts: Add scanCopilot() for session discovery at ~/.copilot/session-state/.
• Tests: Copilot E2E runner paralleling the Codex session runner, updated assertions in existing tests.
• Docs: README, CONTRIBUTING, AGENTS.md updated to mention Copilot as a peer host.

No build pipeline changes needed — Copilot derives from Codex output at setup time. No architectural changes.

Copilot CLI facts

Aspect	Value
Binary	copilot (standalone, not gh copilot)
Install	npm install -g @github/copilot (also: brew, winget, install script)
Config dir	~/.copilot/
Session data	~/.copilot/session-state/{session-id}/
Docs	https://docs.github.com/copilot/how-tos/set-up/install-copilot-cli

Reference implementation

I've put together a working implementation in PR #396. 501 lines added across 14 files, follows the existing Codex/Kiro pattern exactly. Happy to close it if you'd prefer to implement this differently.

Related: #289 (host integration contract RFC) — this would be another concrete data point for that discussion.