Add test coverage for DiffGatherer and comparison retry logic

[ArjenSchwarz]

Context

From code review of #21 (multi-spec comparison feature).

Current Coverage Gaps

• DiffGatherer in internal/comparison/diff.go: 0% coverage - completely untested
• Comparator.Compare retry logic: Only ~30% coverage - retry paths not fully exercised
• Template helper functions (add, sub): 0% coverage (trivial, low priority)

Suggested Improvements

1. Add unit tests for DiffGatherer that mock git operations
2. Add tests that exercise the JSON validation retry loop in Compare
3. Consider integration tests with real git operations (in CI environment)

Priority

Medium - the code works but lacks test coverage for important paths.