Port Antigravity payload enhancements from CLIProxyAPI v6.6.89: - Add ANTIGRAVITY_SYSTEM_INSTRUCTION with agent identity/guidelines - Inject systemInstruction with role 'user' for Antigravity requests - Add requestType: 'agent' to wrapped request body - Update userAgent to 'antigravity' for Antigravity requests This fixes RESOURCE_EXHAUSTED (429) errors when using Antigravity. Adapted from: https://github.com/NoeFabris/opencode-antigravity-auth/pull/137 Reference: https://github.com/router-for-me/CLIProxyAPI/commit/67985d8