perf: reduce LLM input tokens by ~50% via prompt deduplication

- Remove JSON appendix from user_prompt (saves ~3000 chars, 85% reduction)
- Consolidate identity: remove English system_prompt identity, merge into agent_prompt Chinese identity
- Simplify system_prompt output rules: keep only Language Requirement, remove Answer Rules (duplicate with agent_prompt)
- Enhance follow-up context: include more divination_derived fields (changing_yaos, fushen, wu_xing, etc.)
- Fix bug: lowerName -> lower_name in user_prompt (Pydantic snake_case)
- Update tests to reflect new prompt structure

backend/src/core/agentscope/runtime/runner.py

@@ -321,16 +321,27 @@ class AgentScopeRunner:
         run_input: RunAgentInput,
         derived_divination: DerivedDivinationData | None,
     ) -> list[Msg]:
+        if derived_divination is not None:
+            user_text = build_divination_user_prompt(derived=derived_divination)
+        else:
+            _, latest_user_text = extract_latest_user_payload(run_input)
+            user_text = latest_user_text
+
+        if derived_divination is not None and context_messages:
+            last = context_messages[-1]
+            if last.role == "user":
+                context_messages[-1] = Msg(
+                    name=last.name,
+                    role=last.role,
+                    content=user_text,
+                )
+                return context_messages
+
         if context_messages:
             last = context_messages[-1]
             if last.role == "user":
                 return context_messages
 
-        _, latest_user_text = extract_latest_user_payload(run_input)
-        if derived_divination is None:
-            user_text = latest_user_text
-        else:
-            user_text = build_divination_user_prompt(derived=derived_divination)
         user_blocks = [{"type": "text", "text": user_text}]
         if (
             user_blocks