Add factions property to Truths

[rsek]

SI truths include a section for related faction ideas. This is formatted as a single paragraph in the original text:

A seafaring tribe seeks to locate and unite scattered people displaced during the Sundering. An order of sorcerers exploits the chaotic forces that caused the cataclysm.

I this would be useful presented as an array of strings: one string per faction/sentence.

[rsek]

Instead of a string array, I've added it as an array of a new object schema, EntityPrompt, with the shape {text: MarkdownString}. This a placeholder for later expansion with #23 .