Centralize shared D&D LLM assets

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

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+Use the most specific supported in-world character or creature identity for
+each actor or participant. Do not identify a human player, transcript speaker,
+or the GM as an out-of-world person when an in-world identity is supported.
+
+Player, party, glossary, campaign, and NPC registry references may disambiguate
+an identity, but a reference alone cannot establish that the identity
+participated in the transcript.