ENH: Add SkullStrip remote module CID blobs (4 files, 11.25 MiB)

Stages 4 binary blobs referenced by .cid key files in the ITKSkullStrip
remote module so ExternalData can resolve them against
ITKTestingData/CID/.

Every blob was verified before staging by re-hashing to its legacy
SHA-512 or MD5 key and by recomputing its CIDv1-raw (sha2-256)
identifier from file contents. Zero mismatches.

Source: local $ExternalData_OBJECT_STORES and ITKTestingData/MD5/ tree.
No network fetches required.

Assisted-by: Claude Code -- bulk copy + verification pipeline

Author: hjmjohnson