feat(hosts): add support for antigravity agent host

[7H0M45-4N70NY]

Integrated Antigravity host support into gstack, updated setup script, and added documentation. This integration allows Antigravity users to install and manage gstack skills seamlessly on Windows.

[time-attack]

@16francej — recommend closing this PR. It is superseded by #2134, and its executable name and skill roots do not match the currently supported Antigravity setup.

[7H0M45-4N70NY]

Thanks for the update! I agree that #2134 provides a cleaner declarative host configuration and is the right direction overall.

That said, my latest commit (af5e8fd) adds --local support for the Antigravity host in the setup script. From the recent feedback on #2134, it looks like the runtime installation/setup path may still need to be completed, so there may be some overlap there.

From my current understanding, agy targets the Antigravity CLI, while the setup changes in this PR are intended to support the Antigravity IDE installation flow. That said, the end result may ultimately be the same, since the IDE and CLI appear to share the same underlying primitives for skills and subagents. I'll verify both implementations again to ensure they're aligned and that everything is working smoothly.

If my changes are fully covered by #2134 once the setup/runtime path is complete, I'm happy to close this PR. Otherwise, if there's still a gap, I'm happy to contribute the remaining setup changes there instead.