docs(ai): replace deprecated Gemini CLI example with Antigravity CLI

### Description

## Summary

The AI agents documentation still contains instructions for installing and using the Gemini CLI, which has since been deprecated in favor of Antigravity CLI.

This affects the AI agents example page:

- `website/content/en/docs/examples/ai.md`

## Proposed change

Replace the Gemini CLI section with Antigravity CLI by updating:

- installation commands
- executable/command name
- authentication instructions (if needed)
- "See also" link to the Antigravity CLI repository/documentation

This will keep the documentation up to date and prevent new users from following deprecated instructions.

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