feat: add GitHub Copilot CLI support

[ridermw]

Summary

Add first-class GitHub Copilot CLI support to gstack. All 28 skills now work with the standalone Copilot CLI (copilot binary, GA February 2026) alongside the existing Claude Code and Codex support.

This is a purely additive change — no existing Claude Code or Codex functionality is altered.

Related: #289 (host integration contract RFC). This PR follows the existing Codex pattern for pragmatic incremental support. If the adapter abstraction from #289 lands, Copilot support can be migrated onto it — the changes here are scoped and self-contained.

Motivation

The GitHub Copilot CLI went GA in February 2026 as a standalone binary, separate from the old gh copilot extension. It has its own config directory (~/.copilot/), session storage (~/.copilot/session-state/{id}/), and skill discovery. gstack should support it as a first-class host so users on Copilot CLI get the same experience as Claude Code and Codex users.

Architecture decisions

• Copilot shares .agents/skills/ with Codex. Both are non-Claude hosts using the same skill discovery path. Only --host codex is run during generation; Copilot-specific paths (~/.copilot/skills/gstack) are rewritten at setup --host copilot time via sed.
• Build only runs --host codex, not --host copilot. Running both would overwrite the same .agents/skills/ directory. This avoids the conflict entirely.
• Session scanning walks subdirectories. Copilot CLI stores sessions as ~/.copilot/session-state/{session-id}/ directories containing events.jsonl and workspace.yaml, not flat files. scanCopilot() walks these subdirs.
• E2E runner forwards auth. Spawning copilot -p with a temp HOME loses login state. The runner copies config.json and hosts.json from the real ~/.copilot/ to the temp HOME, and GH_TOKEN passes through via env.

Copilot CLI facts

Aspect	Value
Binary	copilot (standalone, not gh copilot)
Install	npm install -g @github/copilot (also: brew, winget, install script)
Config directory	~/.copilot/ (overridable via COPILOT_HOME)
Session data	~/.copilot/session-state/{session-id}/
Interactive mode	copilot
One-shot mode	copilot -p "prompt"
Requirements	Node.js 22+, npm 10+, GitHub Copilot subscription
Docs	https://docs.github.com/copilot/how-tos/set-up/install-copilot-cli

Files changed (14)

Core runtime (3 files)

File	What changed
scripts/gen-skill-docs.ts	Added 'copilot' to Host type, HOST_PATHS, preamble bash. Broadened host === 'codex' gates to include copilot.
setup	Added --host copilot with auto-detection via command -v copilot. Full install section copies .agents/skills/ output and rewrites paths via sed.
bin/gstack-global-discover.ts	Added scanCopilot() — walks ~/.copilot/session-state/{id}/ subdirectories for events.jsonl and workspace.yaml.

Build & CI (2 files)

File	What changed
package.json	Added test:copilot scripts. Build does NOT include --host copilot (Codex output covers both).
.github/workflows/skill-docs.yml	No separate --host copilot CI check (Codex check covers shared .agents/skills/).

Documentation (5 files)

File	What changed
README.md	Added Copilot CLI install section and first-class treatment alongside Claude and Codex.
AGENTS.md	Updated gen command reference (Copilot derives from Codex at setup time).
CONTRIBUTING.md	Renamed "Dual-host" → "Multi-host". 4-column comparison table (Claude | Codex | Copilot). Copilot generation, testing, and new-skill guidance.
CLAUDE.md	Updated prereqs to reference standalone Copilot CLI.
CHANGELOG.md	Added 0.11.10.0 entry (user-facing style).

Tests (4 files)

File	What changed
test/copilot-e2e.test.ts	New. E2E test checking copilot --version availability and spawning with skill context.
test/helpers/copilot-session-runner.ts	New. Subprocess runner for copilot -p. Copies auth config from real ~/.copilot/ to temp HOME.
test/gen-skill-docs.test.ts	Updated assertions to expect command -v copilot in auto-detect output.
test/global-discover.test.ts	Added copilot property expectations to session scan results.

Versioning (1 file)

File	What changed
VERSION	0.11.9.0 → 0.11.10.0 (upstream took 0.11.9.0 for Codex skill loading fix).

Testing

bun test   # all copilot-specific assertions included in static tests

Copilot E2E tests (test/copilot-e2e.test.ts) require the copilot binary installed and authenticated. They skip gracefully when unavailable.

How to verify

# Check Copilot is recognized as a host
./setup --host copilot

# Verify auto-detection picks up copilot
./setup --host auto   # should list copilot if binary is installed

# Run static tests
bun test

[ayovev]

Ha I was just about to spin up a similar PR but you beat me to it; thank you!