Centralize scene planning and description assets

assets/dnd/scenes/prompts/instructions.md

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+Cover the complete provided transcript from its first source unit to its last
+source unit. Return scenes in source-unit order with no gaps or overlaps. Use
+only positive integer source-unit IDs from the transcript, and give every scene
+one inclusive `start_unit_id` and one inclusive `end_unit_id`.
+
+Return exactly one JSON object and no explanatory text. The object must contain
+only a non-empty `scenes` array. Each scene object must contain only
+`start_unit_id` and `end_unit_id`.