Add missing encoding

[pamelafox]

Purpose

This pull request updates file handling in the summarize_results and diff_directories functions within src/evaltools/review/utils.py to explicitly specify UTF-8 encoding. This ensures consistent behavior when reading files, particularly in environments with non-default encodings like Windows.

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Run diff and summary