The content-boundaries guide documents the narrative field

Acceptance Criteria

• The section keeps its three tests and adds the narrative-field case, with the food-blog layout (name, image and times; the story; a card with ingredients and method) as its example
• The "What goes where" table notes that a story may sit inside a rune that declares a narrative field, wrapped in an editorial rune
• The guide states the two rules: one narrative field per rune, never mapped to structured data, and that loose prose inside the rune is still outside its content model
• The composed-runes guide shows a definition with a story field and a template placing it
• npm run format:md:check passes