Centralize NPC LLM assets

assets/dnd/npcs/extract/prompts/instructions.md

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+For every NPC record, return only the observed display name and transcript
+units that support that identity.
+
+Return no other details or lore inferred from general D&D knowledge. Do not
+invent a label for an anonymous creature, crowd, or generic role.
+
+Preserve observed display spelling. Return at least one narrow source range for
+every record.