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/prompt_context.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 from dataclasses import dataclass
 from typing import Dict, List, Optional
 
+from app.agents.chat.interview_turn_plan import InterviewTurnPlan
 from app.agents.state_schema import KnownFact, PersonaThread
 
 
@@ -27,11 +28,20 @@ class ChatPromptContext:
     known_facts: List[KnownFact] | None = None
     persona_threads: List[PersonaThread] | None = None
     recent_questions: List[str] | None = None
+    turn_plan: InterviewTurnPlan | None = None
 
     def guided_system_prompt(self) -> str:
         """用户原话仅以对话历史 + HumanMessage 注入模型。"""
+        from app.agents.chat.interview_turn_plan import (
+            format_interview_turn_directive_block,
+        )
         from app.agents.chat.prompts_conversation import get_guided_conversation_prompt
 
+        directive = (
+            format_interview_turn_directive_block(self.turn_plan)
+            if self.turn_plan is not None
+            else ""
+        )
         return get_guided_conversation_prompt(
             current_stage=self.current_stage,
             empty_slots=self.empty_slots,
@@ -48,4 +58,5 @@ class ChatPromptContext:
             known_facts=self.known_facts or [],
             persona_threads=self.persona_threads or [],
             recent_questions=self.recent_questions or [],
+            turn_directive_block=directive,
         )