Add Synthetic provider support

src/agents/model-auth.ts

@@ -148,6 +148,7 @@ export function resolveEnvApiKey(provider: string): EnvApiKeyResult | null {
     openrouter: "OPENROUTER_API_KEY",
     moonshot: "MOONSHOT_API_KEY",
     minimax: "MINIMAX_API_KEY",
+    synthetic: "SYNTHETIC_API_KEY",
     mistral: "MISTRAL_API_KEY",
     opencode: "OPENCODE_API_KEY",
   };