[dandye]

Summary

• Add cns:// and /cns/... usecase source retrieval support in src/logstory/logstory.py via fileutil (_normalize_cns_path, _CnsBlob, _get_cns_blobs).
• Support optional LOGSTORY_GFS_USER role/group impersonation (--gfs_user) for group-owned Colossus paths.
• Update DEFAULT_BUCKET to cns:///cns/is-d/home/dandye/logstory_usecases_20260918.
• Add Justfile recipes for publishing/listing usecases on CNS (usecase-publish-cns, usecase-publish-all-cns, usecase-list-cns) and registering dataset variants in DataNexus via mlc (datanexus-register-dryrun, datanexus-register using AUTO format and latest alias).
• Add unit tests in tests/test_cli.py covering DEFAULT_BUCKET, cns:// URI normalization, _get_cns_blobs directory and recursive file listing, and _CnsBlob downloads.

Test Plan

• uv run pytest -q (177 passed)
• uv run ruff check src/ tests/
• uv run pyink --check src/ tests/
• Live verification with uv run logstory usecases list-available and uv run logstory usecases get NETWORK_ANALYSIS