docs: add review comment checklist

Author: cschanhniem

docs/REVIEW-COMMENT-CHECKLIST.md

@@ -0,0 +1,23 @@
+# Review Comment Checklist
+
+Open Problem Lab should make review comments actionable.
+
+Use this checklist when leaving a blocking or important review comment.
+
+```markdown
+## Review comment checklist
+
+**Issue type:** source | method | boundary | status | field context | generated output | structure.
+
+**What must change:** One concrete action.
+
+**Why it matters:** One sentence.
+
+**Suggested fix:** Optional path forward.
+
+**Blocks merge?** yes | no | only blocks confidence upgrade.
+```
+
+## Acceptance test
+
+A review comment is useful when the contributor can tell what to change, why it matters, and whether it blocks merge.