Instead of creating a new progress bar for each path, count total files
across all paths upfront and use a single unified progress bar. This
provides clearer UX when processing multiple directories.
- Add -q/--quiet flag to suppress all output except errors
- Add progress bar with 250ms refresh, file count, and ETA display
- Print summary report to stderr on completion (files processed, skipped, failed, bytes, duration)
- Support reading paths from stdin with "-" argument (e.g., find | attrsum sum add -)
- Update README with new features and updated TODO section
