feat(chat): server-side interview turn plan (mode, anchor slot, snippet)

- Add plan_interview_turn: emotion_first / memoir_push / follow_user_only
- Inject hard directive block at top of guided system prompt
- Pass stage_switched_this_turn from ChatOrchestrator after stage detection
- Log interview_turn_plan for observability; add unit tests

api/app/agents/chat/interview_agent.py

@@ -15,6 +15,7 @@ from app.agents.chat.interview_state_hints import (
     extract_recent_questions,
     update_recent_questions,
 )
+from app.agents.chat.interview_turn_plan import plan_interview_turn
 from app.agents.chat.personas import normalize_interview_persona
 from app.agents.chat.prompt_context import ChatPromptContext
 from app.agents.chat.prompts_conversation import (
@@ -99,6 +100,7 @@ class InterviewAgent:
         occupation: str = "",
         profile_birth_year: int | None = None,
         profile_era_place: str = "",
+        stage_switched_this_turn: bool = False,
     ) -> AgentChatTurn:
         """生成状态感知的访谈回复，不持久化（由 Orchestrator 负责）"""
         if not self.llm:
@@ -133,6 +135,20 @@ class InterviewAgent:
             max_tokens = int(settings.chat_interview_max_tokens)
             max_chars = int(settings.chat_interview_max_chars_per_segment)
 
+            turn_plan = plan_interview_turn(
+                current_stage=memoir_state.current_stage,
+                empty_slots=empty_slots,
+                normalized_user_message=text_for_model,
+                memory_evidence_text=memory_evidence_text,
+                stage_switched_this_turn=stage_switched_this_turn,
+            )
+            logger.info(
+                "event=interview_turn_plan mode={} anchor_slot={} snippet_len={}",
+                turn_plan.mode,
+                turn_plan.anchor_slot_key or "-",
+                len(turn_plan.anchor_snippet or ""),
+            )
+
             ctx = ChatPromptContext(
                 current_stage=memoir_state.current_stage,
                 empty_slots=empty_slots,
@@ -149,6 +165,7 @@ class InterviewAgent:
                 known_facts=memoir_state.known_facts,
                 persona_threads=memoir_state.persona_threads,
                 recent_questions=recent_questions or memoir_state.recent_questions,
+                turn_plan=turn_plan,
             )
             system_prompt = ctx.guided_system_prompt()
             messages: List[Any] = [SystemMessage(content=system_prompt)]