Escape Hint
Per-StructuredEntry policy that tells each format backend how to serialize the entry's value. Lives on StructuredEntry rather than on the StructuredScalar itself so that the same scalar type can be emitted differently in different output contexts. Exists because the two existing hardcoded transformers disagree on scalar escaping: SkillTransformer emits the skill `description` raw in YAML frontmatter, while AgentTransformer wraps the agent `description` in a YAML-safe double-quoted scalar. Both are valid for their case — the difference belongs to the transformation instance, not to the backend.
