#234 data delete fails with "Directory not empty (os error 39)" when concurrent writes are active

Summary

swamp data delete <model> <data_name> --force fails with os error 39 (Directory not empty) when another process (e.g. a running workflow) is concurrently writing new versions to the same data artifact.

Steps to Reproduce

1. Start a workflow that writes data to a model (e.g. a workflow with multiple steps writing to a command/shell model)
2. While the workflow is running, attempt to delete all versions of that data:

   swamp data delete <model> <data_name> --force

3. The delete acquires the lock, begins removing versions, but new files appear between deletion and directory removal

Expected Behavior

The delete should either:

• Retry the rmdir after a brief pause
• Skip newly-created versions and report partial deletion
• Document that concurrent deletes are best-effort

Actual Behavior

FTL error Error: "Directory not empty (os error 39): remove '/path/to/.swamp/data/.../log'"

Notes

• Deleting a specific version (--version N --force) works correctly even under concurrent writes
• Deleting all versions works correctly when no concurrent process is writing
• The --json output mode for successful deletes is clean: {modelId, modelName, dataName, versionsDeleted}
• Error messages for non-existent data/model/version are clear and helpful

Environment

• CLI: 20260504.233645.0-sha.430c1535
• OS: Linux x86_64