Cloud Storage Compatibility (fsspec) Throughout

Replace all os.path, pathlib.Path, glob, and shutil I/O in processing stages with fsspec-backed equivalents so S3, GCS, and Azure paths work identically to local paths.

Why: All production data lives in object storage. Today, Curator's download stages support cloud I/O but the processing stages use local-only primitives, breaking on s3:// or gs:// URIs. This forces users to sync data locally before processing — untenable at large scale.
Definition of Done:

• All file reads/writes in stages/ use fsspec.open() or equivalent
• s3://, gs://, az://, hf://, and file:// paths work in all stages without code changes
• FilePartitioningStage resolves cloud paths correctly (issue #1568)
• CI tests added for S3-compatible storage (e.g., MinIO in CI)
• No dependency on os.path, pathlib.Path, glob, or shutil for URI handling in stage code (enforced by linter rule per issue #984)

Start by inventorying URI handling throughout stages/ and read FilePartitioningStage alongside issue #1568. Then inspect the CI setup for adding MinIO coverage. Done means all listed URI schemes work across stages, local-only path primitives are absent from stage URI handling, and the linter requirement is enforced.