feat: enhance auto-assign reviewer script with CODEOWNERS support

- Add CODEOWNERS-aware file categorization
- Refactor script from 333 to 255 lines (23% reduction)
- Add comprehensive test coverage including CODEOWNERS test
- Improve logging and error handling
- All 20 tests passing

Signed-off-by: Venky Ganesh <[email protected]>

Author: venkywonka