feat: Add support for Antigravity AI agent

[NotArsal]

This PR adds first-class support for the Antigravity agent. It includes the new host configuration and updates to README.md and .gitignore.

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[NotArsal]

/trunk merge

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[time-attack]

@16francej — recommend closing this PR as superseded by #2134. This near-duplicate relies on an unsupported executable and a weaker host identity in the shared Gemini namespace, whereas #2134 owns the official Agy identity and documented global and workspace roots. Credit #2187 there and retain any genuinely distinct tests or naming rationale.