Add concurrency limit of 4 to sync fetch operations
Limits parallel network operations to avoid overwhelming remotes:
- Push to checkouts: max 4 concurrent
- Fetch from remotes: max 4 concurrent
- Fetch verse remotes: max 4 concurrent
Remote push was already limited to 2 concurrent operations.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>