fix: implement Export command, date function, and version bumping

- Implement cmd_export() for CSV/JSON export of manifest data
- Replace hardcoded date placeholder with actual system time calculation
  using Howard Hinnant's algorithm for Unix timestamp conversion
- Implement bump-patch, bump-minor, bump-major Justfile recipes
  for semantic version management

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Jonathan D.A. Jewell