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+.

AGENTS.md

@@ -53,7 +53,10 @@ Three files do everything:
   - Upstream dispatch:
     - `referenceImages` present → `POST {baseURL}/images/edits` as
       `multipart/form-data` (blobs decoded from data URLs via
-      `decodeDataUrl` → `Uint8Array<ArrayBuffer>`).
+      `decodeDataUrl` → `Uint8Array<ArrayBuffer>`). Single reference uses
+      field name `image`; **two or more references use `image[]`** to
+      match OpenAI's documented array syntax — strict gateways reject
+      repeated `image` parts with a `duplicate_parameter` 400.
     - Otherwise → `POST {baseURL}/images/generations` as JSON.
     - Always sends `stream: true, partial_images: 2` first. On a 400 that
       mentions `stream` or `partial_images` (see