Centralize spell and combat-turn assets

assets/dnd/combat-turns/prompts/instructions.md

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+Return the combat_turns array even when no combat turn is established. Return
+actor, turn_kind, and source_refs for every record. For turn_kind, use exactly
+one of: turn, reaction, legendary_action, lair_action, or other. Cite the
+transcript ranges that establish both the actor and the combat event. Use the
+players, party, and transcript context to map speakers to in-world actors. NPC
+names may help disambiguate identity but do not replace transcript evidence.