docs: add Antigravity (Agy) integration guide

docs/ADDING_A_HOST.md

@@ -1,7 +1,7 @@
 # Adding a New Host to gstack
 
 gstack uses a declarative host config system. Each supported AI coding agent
-(Claude, Codex, Factory, Kiro, OpenCode, Slate, Cursor, OpenClaw) is defined
+(Claude, Codex, Factory, Kiro, OpenCode, Slate, Cursor, OpenClaw, Hermes, GBrain, Agy) is defined
 as a typed TypeScript config object. Adding a new host means creating one file
 and re-exporting it. Zero code changes to the generator, setup, or tooling.
 
@@ -17,6 +17,9 @@ hosts/
 ├── slate.ts         # Slate (Random Labs)
 ├── cursor.ts        # Cursor
 ├── openclaw.ts      # OpenClaw (hybrid: config + adapter)
+├── hermes.ts        # Hermes
+├── gbrain.ts        # GBrain
+├── agy.ts           # Google Antigravity
 └── index.ts         # Registry: imports all, derives Host type
 ```
 

docs/AGY.md

@@ -0,0 +1,71 @@
+# Google Antigravity (Agy) Integration
+
+gstack supports Google Antigravity (Agy) as a first-class agent host. This integration allows you to generate and run Agy-compatible skills, load them dynamically via the `agy` plugin system, and benchmark agy models alongside Claude, OpenAI, and Gemini.
+
+## Features
+
+- **Automatic Setup**: Installs gstack skills directly into the Antigravity config directory via `./setup --host agy`.
+- **Agy Plugin Compatibility**: Packages skills as a standard Agy plugin with a `plugin.json` manifest, automatically registered with `agy plugin install`.
+- **Exclusion of Incompatible Skills**: Automatically filters out Claude-specific wrappers (like `gstack-codex`) from Agy generation.
+- **Benchmark Integration**: Register Agy as a benchmark provider using `agy --print` to trace latency, quality, and tokens.
+
+---
+
+## Setup & Installation
+
+To generate and install gstack skills into your local Antigravity environment:
+
+```bash
+./setup --host agy
+```
+
+### What Setup Does
+
+1. **Precompile & Prepare**: Ensures the browse tool and other runtime dependencies are compiled.
+2. **Generate Skills**: Runs `bun run gen:skill-docs --host agy` to generate Agy-compatible frontmatter.
+3. **Initialize Plugin**: Creates the plugin directory structure under `.gemini/config/plugins/gstack`.
+4. **Symlink/Copy Assets**: Places necessary executables (`bin`, `browse`, `review`, `qa`) into the plugin root.
+5. **Register Plugin**: Invokes `agy plugin install` to import the gstack plugin into `~/.gemini/config/import_manifest.json`.
+
+---
+
+## Plugin Structure
+
+When installed, the gstack plugin resides at:
+`~/.gemini/config/plugins/gstack/`
+
+The directory layout matches the standard Agy plugin structure:
+
+```
+~/.gemini/config/plugins/gstack/
+├── plugin.json         # Manifest declaring name, version, and license
+├── skills/             # Generated Agy-compatible skills
+│   ├── gstack/         # Root gstack skill (/gstack)
+│   ├── gstack-qa/      # QA testing skill (/qa)
+│   └── ...
+├── bin/                # Shared executables and CLI tools
+└── browse/             # Browse tool distribution files
+```
+
+---
+
+## Benchmarking with Agy
+
+Agy is registered as a model benchmark provider. You can execute prompts against Agy using the gstack benchmark runner.
+
+### Pre-requisites
+- The `agy` CLI must be installed and available in your `PATH`.
+- Credentials must be configured in `~/.gemini/oauth_creds.json`.
+
+### Run Benchmark
+To run a dry run and check availability:
+```bash
+gstack-model-benchmark --models agy --dry-run
+```
+
+To execute a benchmark run:
+```bash
+gstack-model-benchmark --models agy --prompt "Explain quantum computing in one sentence."
+```
+
+The runner automatically passes `--dangerously-skip-permissions` to the CLI to run headless without hanging on interactive permission prompts.