Gemini CLI no longer works — migrate to Antigravity CLI #849

Problem

The Gemini CLI integration no longer works. Google has migrated Gemini CLI to the new Antigravity CLI, so the old gemini CLI commands/auth path are broken.

Proposed fix

Migrate the Gemini provider integration to use the Antigravity CLI instead of the deprecated Gemini CLI.

Reference

Migration guide: https://medium.com/google-cloud/migrating-to-antigravity-cli-a841c6964f37

Expected behavior

Automaker should invoke the Antigravity CLI for Gemini-backed agents/models, with the corresponding auth/setup flow updated accordingly.