Replace programs.gemini-cli.settings with programs.antigravity-cli.settings

Now `home-manager switch` outputs: ``` warning: The option `programs.gemini-cli.settings' defined in `/nix/store/6ar2vjfzlhxwqqzhd8sjxkjp9dyh9s78-source/modules/home-manager/gemini-cli.nix' has been renamed to `programs.antigravity-cli.settings' ```