feat(#786): elevate GitHub Copilot installer — lifecycle hook + AGENTS.md (#804)

* feat(#786): elevate Copilot installer with lifecycle hook + AGENTS.md

Emit a self-contained sessionStart hook config (.github/hooks/gsd-session.json
local, ~/.copilot/hooks/gsd-session.json global) and write AGENTS.md at the repo
root (Copilot CLI reads it as primary instructions) alongside
copilot-instructions.md. The hook is an inline `command` hook (no separate hook
script), so it cannot dangle. Uninstall removes both and preserves user content.

Verified against GitHub Copilot CLI primary docs: hooks-configuration (camelCase
events, version+hooks shape, inline bash/powershell command hooks) and
add-custom-instructions (AGENTS.md read at repo root as primary instructions).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#786): set changeset pr number to 804

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -800,7 +800,7 @@ The migration-specific ownership and source snapshots live in
 | Kilo | `~/.config/kilo` | `./.kilo` | `command/gsd-*.md` | `agents/gsd-*.md` | `kilo.json` or `kilo.jsonc`; no GSD hooks |
 | Gemini CLI | `~/.gemini` | `./.gemini` | `commands/gsd/*.toml` | `agents/gsd-*.md` | `settings.json` feature flag, hooks, and statusline |
 | Codex | `~/.codex` | `./.codex` | `skills/gsd-*/SKILL.md` | `agents/` source markdown plus per-agent TOML | `config.toml` `[agents.gsd-*]`, `[features].hooks` (canonical; legacy alias `codex_hooks` is recognized and migrated forward on reinstall, #3566), and hook tables |
-| GitHub Copilot | `~/.copilot` | `./.github` | `skills/gsd-*/SKILL.md` and `copilot-instructions.md` | `.agent.md` files | No GSD hooks or statusline |
+| GitHub Copilot | `~/.copilot` | `./.github` | `skills/gsd-*/SKILL.md`, `copilot-instructions.md`, and `AGENTS.md` (repo root, local) | `.agent.md` files | Self-contained `sessionStart` hook (`hooks/gsd-session.json`, inline `command` type); no statusline |
 | Antigravity | auto-detected: `~/.gemini/antigravity`, `~/.gemini/antigravity-ide`, or `~/.gemini/antigravity-cli` | `./.agent` | `skills/gsd-*/SKILL.md` | `agents/gsd-*.md` | Gemini-style `settings.json` hook entries when installed by GSD |
 | Cursor | `~/.cursor` | `./.cursor` | `skills/gsd-*/SKILL.md` | `agents/gsd-*.md` | Rule references under `rules/`; no GSD hooks |
 | Windsurf | `~/.codeium/windsurf` | `./.windsurf` | `skills/gsd-*/SKILL.md` | `agents/gsd-*.md` | Rule references under `rules/`; no GSD hooks |

docs/how-to/install-on-your-runtime.md

@@ -156,6 +156,13 @@ npx @opengsd/gsd-core@latest --copilot --global
 
 Skills land in `~/.copilot/`. GSD installs as agent `.md` files and repository instruction files.
 
+GSD also wires Copilot's lifecycle hooks and instruction files:
+
+- **`AGENTS.md`** (local installs) — written at the repository root, which GitHub Copilot CLI reads as primary instructions, alongside `copilot-instructions.md`.
+- **Lifecycle hook** — a `sessionStart` hook config is written to `.github/hooks/gsd-session.json` (local) or `~/.copilot/hooks/gsd-session.json` (global). It is a self-contained inline `command` hook (no separate hook script to install), so it can never reference a missing script. The hook is advisory-only: at session start it surfaces whether the project has a `.planning/` workflow.
+
+Both are removed (and any user-authored content preserved) on `--uninstall`.
+
 **Override the install directory:**
 
 ```bash