docs: explain grading-schema overrides

[tomatotomata]

Closes #951

Summary

• document the per-fixture grading-schema.json override in the release-prepare eval README
• link the real step-1-plan/fixtures/grading-schema.json example
• clarify that grading-schema.json controls exact-versus-rubric prose comparison while assertions.json supplies deterministic predicates

Validation

• git diff --check
• PYTHONPATH=tools/skill-evals/src python -m pytest tools/skill-evals/tests/test_runner.py -k grading_schema -q (5 targeted tests passed; the CLI-grader case is excluded)
• the Windows CLI-grader test remains blocked by its existing shell-command fixture (WinError 2)