Add Antigravity (agy) as a third agent backend for gemini-* models

mac/Sources/Byre/Composer.swift

@@ -18,6 +18,12 @@ let models = [
     "gpt-5.6-sol",
     "gpt-5.6-terra",
     "gpt-5.6-luna",
+    "gemini-4-argon-high",
+    "gemini-4-argon-medium",
+    "gemini-4-argon-low",
+    "gemini-3.8-flash-high",
+    "gemini-3.8-flash-medium",
+    "gemini-3.8-flash-low",
 ]
 
 struct PendingImage: Identifiable {
@@ -69,7 +75,13 @@ func uploadImages(_ api: API, _ imgs: [PendingImage]) async throws -> [String]?
 var modelChoices: [String: String] = [:]
 
 func vendorOf(_ model: String) -> String {
-    model.hasPrefix("gpt-") ? "codex" : "claude"
+    if model.hasPrefix("gpt-") {
+        return "codex"
+    }
+    if model.hasPrefix("gemini-") {
+        return "antigravity"
+    }
+    return "claude"
 }
 
 @MainActor