Add installation and configuration documentation for Antigravity CLI

### Describe the feature or problem you’d like to solve

Google recently announced the transition from Gemini CLI to the new Antigravity CLI, with the deprecation of Gemini CLI for free and Pro/Ultra tiers taking effect on June 18, 2026.

Currently, the official repository documentation in `docs/installation-guides/install-antigravity.md` only covers installation steps for the Antigravity IDE. While the core MCP configuration format is similar, Antigravity CLI uses completely different configuration file paths and directories compared to the desktop IDE. Without explicit documentation for the CLI, users migrating from Gemini CLI will struggle to locate where to place their `mcp_config.json` files.

### Proposed solution

We propose updating the documentation to explicitly include setup instructions for the **Antigravity CLI**, either by adding a dedicated section to `install-antigravity.md` or creating a standalone guide.

The documentation should highlight the correct configuration paths and parameters unique to the CLI:

#### 1. Configuration File Locations for Antigravity CLI

* **Global Configuration:** `~/.gemini/antigravity-cli/mcp_config.json`
* **Workspace Configuration:** `.agents/mcp_config.json`

#### 2. Configuration Setup Options

The guide should provide both setup methods compatible with the CLI architecture:

##### Option 1: Remote Server Configuration

```json
{
"mcpServers": {
"github": {
"serverUrl": "https://api.githubcopilot.com/mcp/",
"headers": {
"Authorization": "Bearer YOUR_GITHUB_PAT"
}
}
}
}

```

*(Note: Emphasize that `serverUrl` is strictly required instead of `url` or `httpUrl`)*

##### Option 2: Local Docker Server Configuration

```json
{
"mcpServers": {
"github": {
"command": "docker",
"args": [
"run",
"-i",
"--rm",
"-e",
"GITHUB_PERSONAL_ACCESS_TOKEN",
"ghcr.io/github/github-mcp-server"
],
"env": {
"GITHUB_PERSONAL_ACCESS_TOKEN": "YOUR_GITHUB_PAT"
}
}
}
}

```

#### 3. CLI Management Commands

Include a mention of the built-in management commands inside the Antigravity CLI terminal interface to check status:

* Use `/mcp` to list and manage active MCP servers.

### Example prompts or workflows (for tools/toolsets only)

*Not applicable as this is a documentation improvement request. However, it enables the following user workflow within the new CLI environment:*

1. User initializes a project using Antigravity CLI.
2. User runs `/mcp` to check connected servers, notices GitHub is missing.
3. User follows the updated guide to create `.agents/mcp_config.json` in their repository root.
4. User runs a terminal command like: `Please review my latest open PRs on GitHub and summarize the changes.`

### Additional context

For more details on the ecosystem migration, architecture, and configuration differences, please see the official resources below:

* **Official Migration Announcement:** [An Important Update: Transitioning Gemini CLI to Antigravity CLI](https://developers.googleblog.com/an-important-update-transitioning-gemini-cli-to-antigravity-cli/)
* **Community Discussion & Lifecycle Timeline:** [Gemini CLI GitHub Discussion #27274](https://github.com/google-gemini/gemini-cli/discussions/27274)
* **Antigravity CLI Usage Guide:** [Using Antigravity CLI Documentation](https://antigravity.google/docs/cli-using)
* **Configuration Migration Reference:** [Gemini CLI to Antigravity CLI Migration Guide](https://antigravity.google/docs/gcli-migration)