[Feature]: Add Antigravity CLI (antigravity-cli) as a provider

[NitayRabi]

Proposal

Opening this as an issue first to gauge interest before drafting an implementation.

I'd like to propose adding Antigravity CLI (antigravity-cli) as a supported provider in T3 Code.

Similar to Claude Code, Codex, and OpenCode, Antigravity operates as a command-line agent capable of conducting repository analysis, editing files, running terminal commands, and orchestrating subagents.

Integrating antigravity-cli would allow T3 Code users to run Antigravity as their backend coding agent directly within the T3 Code GUI.

Suggested Integration Shape

• Antigravity Driver & Adapter: Introduce an AntigravityDriver / AntigravityAdapter to map event streams, tool invocations, and token logs over the WebSocket connection.
• Provider Registration: Register antigravity as a supported provider kind, wiring up standard CLI checks and configurations.
• Verification & Lifecycle: Handle startup checks (verifying antigravity-cli is installed and authenticated) and gracefully capture stream failures.
• Documentation: Create a standard provider guide under docs/providers/antigravity.md.

Let me know if this is something you'd be open to supporting.

---

🤖 Filed with Antigravity CLI

[NitayRabi]

This suggestion aligns with the discussion started in #1983 (comment) regarding adding support for Antigravity.