test: make reference-link validator assertions portable on Windows

On Windows, scripts/validate-reference-links-test.js fails on the default branch because the validator formats path.relative() with backslashes while the regression assertion only accepts forward slashes. Reproduction: run node --test scripts/validate-reference-links-test.js on Windows; the test 'fails when a skill links the shared checklist as if it were colocated' reports 1 failure at line 75. Update the assertion to accept platform-native separators (or normalize the reported path) while preserving the same failure check. The validator behavior is correct; this is a cross-platform test portability defect.