refactor(editor-ext): remove unused setImageCaption command (F7)

The setImageCaption command and its Commands<> declaration were dead:
captions are written via the generic updateAttributes in
useImageTextFieldControl, and a repo-wide grep finds zero callers.
Remove the speculative implementation (image.ts) and its type
declaration.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/editor-ext/src/lib/image/image.ts

@@ -55,7 +55,6 @@ declare module "@tiptap/core" {
       setImageAlign: (
         align: "left" | "center" | "right" | "floatLeft" | "floatRight",
       ) => ReturnType;
-      setImageCaption: (caption: string | undefined) => ReturnType;
       setImageWidth: (width: number) => ReturnType;
       setImageSize: (width: number, height: number) => ReturnType;
     };
@@ -194,11 +193,6 @@ export const TiptapImage = Image.extend<ImageOptions>({
         ({ commands }) =>
           commands.updateAttributes("image", { align }),
 
-      setImageCaption:
-        (caption) =>
-        ({ commands }) =>
-          commands.updateAttributes("image", { caption }),
-
       setImageWidth:
         (width) =>
         ({ commands }) =>