fix: update _img_to_bytes docstring to reflect BytesIO return type

Agent-Logs-Url: https://github.com/github/awesome-copilot/sessions/048d704a-d78a-4a46-80d9-a155ff1e0d85

Co-authored-by: dvelton <48307985+dvelton@users.noreply.github.com>

Author: Copilot

skills/eyeball/tools/eyeball.py

@@ -402,7 +402,7 @@ def _stitch_vertical(images, gap=4):
 
 
 def _img_to_bytes(img):
-    """Convert PIL Image to PNG bytes."""
+    """Convert PIL Image to a PNG BytesIO buffer (file-like object)."""
     buf = io.BytesIO()
     img.save(buf, format="PNG")
     buf.seek(0)