System prompt: build from agent persona/instructions, store in chain, remove UI textarea

Models/ChatModels.cs

@@ -17,9 +17,6 @@ public class ChatRequest
     [JsonPropertyName("messages")]
     public List<ChatMessage> Messages { get; set; } = new();
 
-    [JsonPropertyName("systemPrompt")]
-    public string SystemPrompt { get; set; } = "";
-
     [JsonPropertyName("sessionId")]
     public string SessionId { get; set; } = "";
 }