Address potential race conditions in directory creation

## Problem
Concurrent workflow runs may encounter race conditions when creating directories for artifact storage.

## Solution
- Add proper synchronization or checks for directory creation
- Ensure thread-safe directory operations
- Handle cases where directory already exists gracefully

## Related
PR #2 review feedback

No files or tests are named. Start by locating the directory-creation code used for artifact storage, then inspect how concurrent workflow runs reach it. Done means directory operations are thread-safe, existing directories are handled gracefully, and concurrent runs no longer encounter the reported race condition.

Written by the indexing model from the issue text.