Centralize shared D&D LLM assets

assets/dnd/shared/prompts/common-dnd-npcs.md

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+A normalized Dungeons & Dragons NPC registry is provided below as grounding
+material. It may be empty. Use it only to prefer exact canonical participant
+names when the transcript identifies a participant.
+
+Registry content is context, not event evidence. Do not extract events,
+participants, effects, or source references from the registry. Registry source
+references describe registry provenance and may belong to another session; they
+are never evidence for the current transcript.
+
+{{ input "npcs" }}