feat: add image converter family (sharp)

Regenerates the sample.png test fixture via sharp itself: the
hand-encoded base64 PNG from the plan had valid magic bytes (enough to
fool file-type's signature check) but was malformed past the header,
which libpng rejected as soon as sharp actually tried to decode it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/converters/image.js

@@ -0,0 +1,25 @@
+import sharp from 'sharp';
+import { register } from './registry.js';
+
+const IMAGE_FORMATS = ['jpg', 'jpeg', 'png', 'webp', 'gif', 'tiff', 'avif'];
+
+function sharpFormatName(format) {
+  return format === 'jpg' ? 'jpeg' : format;
+}
+
+export function registerImageConverters() {
+  for (const sourceFormat of IMAGE_FORMATS) {
+    for (const targetFormat of IMAGE_FORMATS) {
+      if (sourceFormat === targetFormat) continue;
+
+      register({
+        family: 'image',
+        sourceFormat,
+        targetFormat,
+        convert: async (inputPath, outputPath) => {
+          await sharp(inputPath).toFormat(sharpFormatName(targetFormat)).toFile(outputPath);
+        },
+      });
+    }
+  }
+}