When creating a node on a remote compute, the controller now sends
only the image filename instead of the absolute local path. The remote
compute will search for the image in its own configured images_directories.
If the image is not found, the compute returns ImageMissingError, which
triggers the controller's automatic image upload mechanism.
This fixes issue #2676 where remote computes reject paths from the
controller due to mismatched directory prefixes.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>