feature: agy (antigravity-cli) support

[hardyrafael17]

Did you check the docs?

• I have read all the sidekick.nvim docs

Is your feature request related to a problem? Please describe.

No, it's just the addition of a new cli tool, similar to gemini-cli

Describe the solution you'd like

agy listed in the cli tools available when the shortcut is used a

Describe alternatives you've considered

N/A

Additional context

https://antigravity.google/docs/cli-overview

[ashleyghooper]

Can we get this merged? Gemini CLI is being deprecated so it would be good if sidekick could support its replacement.