Add NPC normalization prompt context

internal/modules/dnd/normalize/npcs/assets/prompts/instructions.md

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+Return duplicate groups only when the transcript context clearly establishes a
+single individual. Prefer no group when identity is ambiguous.
+
+Copy supplied display names into each group's members. Choose canonical_name
+from that same group's supplied members. Prefer a complete stable proper name
+over an abbreviation, but prefer an unadorned proper name over that name plus a
+contextual class, role, title, or relationship descriptor unless the descriptor
+is established as part of the name.
+
+Do not invent names, source references, replacement records, or explanations.