Flatten D&D module prompt asset trees

internal/modules/extract/dnd/spells/assets/prompts/instructions.md

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+Source references must use the source-unit IDs exactly as provided.
+
+Return only D&D spell-cast artifacts. For each spell cast, identify the in-world
+caster, spell name, effect, narrative description, and source references using
+source_id, start_unit_id, and end_unit_id.
+
+Use roster and glossary reference material only to clarify source text. Do not
+return spells, casters, or effects that are mentioned only in reference
+material.
+
+Return exactly one JSON object and no explanatory text.