fix: use image[] field for multi-reference edits

OpenAI documents the multipart array syntax as image[] for multiple
references; strict gateways reject repeated 'image' parts with
duplicate_parameter 400. Use 'image' for single, 'image[]' for 2+.

index.ts

@@ -49,6 +49,7 @@ async function callUpstream(args: {
 			form.append("stream", "true");
 			form.append("partial_images", "2");
 		}
+		const imageField = referenceImages.length > 1 ? "image[]" : "image";
 		for (let i = 0; i < referenceImages.length; i++) {
 			const dataUrl = referenceImages[i];
 			if (!dataUrl) continue;
@@ -56,7 +57,7 @@ async function callUpstream(args: {
 			if (!decoded) continue;
 			const ext = decoded.mime.split("/")[1] ?? "png";
 			form.append(
-				"image",
+				imageField,
 				new Blob([decoded.bytes], { type: decoded.mime }),
 				`ref-${i}.${ext}`,
 			);