Add D&D NPC extraction and validation

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

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+Return exactly one JSON object and no explanatory text.
+
+For every NPC record, cite one or more transcript source-unit ranges that
+collectively support the canonical name, every alias, the description, and
+every relationship. Use integer start_unit_id and end_unit_id values from the
+transcript. Do not provide source_id; the extractor assigns it automatically.
+Use narrow ranges when evidence is not contiguous and do not bridge unrelated
+conversation with a broad range.
+
+A canonical name must be the most specific in-world identity supported by the
+transcript. Never use a human player, transcript speaker, or GM name when an
+associated in-world character or creature is identified. Player, party, and
+glossary references may disambiguate identities, but they are not transcript
+evidence and cannot establish that an NPC appeared, acted, or was discussed.
+Do not return a person or entity mentioned only in those references.
+
+Descriptions must be short session records, not biographies, statistics,
+alignment, motivations, or lore inferred from general D&D knowledge. Do not
+summarize every action or follow a participant through unrelated scenes.
+Relationships must be stated or directly demonstrated by cited transcript
+units, not inferred from game lore.
+
+Return aliases and relationships as arrays, including empty arrays when there
+are none. Return only NPC records supported by the transcript and preserve
+observed display spelling.