Document the grading-schema override in the release-prepare eval README

[justinmclean]

Summary

tools/skill-evals/evals/release-prepare/README.md documents assertions.json
as the only way to grade prose steps, but a per-step grading-schema.json
override also exists and is used in this repo.

Background

The suite's "Grading prose steps" section is where a contributor editing
fixtures looks first. It never mentions grading-schema.json, so someone
hitting a brittle exact-match on a prose field will not know the override exists
and will work around it instead, usually by loosening the expected value until
it passes.

Where to look

• evals/release-prepare/README.md - the "Grading prose steps" section.
  grep -c grading-schema on it currently returns 0.
• evals/release-verify-rc/*/fixtures/grading-schema.json - working examples.
• tools/skill-evals/src/skill_evals/runner.py - how the override is resolved.

Acceptance criteria

• The README mentions grading-schema.json, what it does, and where it goes.
• It points at a real example in the tree.
• It states the relationship to assertions.json.

Estimated effort

~1 hour for someone new to the codebase.