[idea] Antigravity CLI integration support

[ludoo]

idea / problem

Add antigravity-cli (agy) as a first class citizine in herdr. Antigravity-cli is the successor to gemini-cli, and the native cli harness for Gemini.

requested change

Antigravity-cli with feature parity to the other supported harnesses.

why you want this

Because I have a Gemini subscription, and use agy as my harness when I work with Gemini.

Provisional support added here ludoo#1

[ludoo]

Any chance I can open a PR with this? I have it working locally, and it's pretty useful to anyone with a Gemini sub.

[bradshimmin]

Sounds great here. I'd appreciate integration support for Gemini CLI itself, if possible, folks.