feat: resume single-file okdev cp downloads

[acmore]

Summary

• add resumable single-file downloads that continue from existing local bytes across retries and reruns
• probe remote file metadata and stream ranged reads so late retryable errors do not force a full restart
• surface resume progress in the CLI and document that resume currently applies only to single-file downloads

Test Plan

• go test ./internal/kube ./internal/cli
• .venv/bin/pre-commit run --all-files --hook-stage manual okdev-gofmt