Migrate from Gemini CLI to Antigravity CLI

We have `plugin/gemini-extension.json` for Gemini CLI integration. We should remove it and figure out what we need to do to enable our plugin in the new Antivigravity CLI.

Source:
https://developers.googleblog.com/an-important-update-transitioning-gemini-cli-to-antigravity-cli/

Start by inspecting plugin/gemini-extension.json and reading the linked announcement about the transition from Gemini CLI to Antigravity CLI. Determine the required Antigravity integration steps; the work is done when the Gemini configuration is removed and the plugin is enabled for the new CLI.

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