refactor(chat): AI-native prompts, remove interview heuristics

- Drop interview_reply_length and utterance_substance; always run stage LLM
  and memory retrieval when enabled; trim Settings fields and .env.example.
- Replace guided/opening prompts with compact fact blocks plus unified
  behavior guidance; slim background_voice and persona to tone hints.
- InterviewAgent uses fixed chat_interview max_tokens/chars/segments.

Also includes stacked work: profile followup/extract path, evaluation rubric
and judge schema updates, transcript SPLIT handling in execution service,
user export markdown split tests, and golden case fixture.

api/tests/fixtures/evaluation_golden_cases.json

@@ -0,0 +1,39 @@
+{
+  "description": "金标准用户轮次：导入 regression set 后通过 POST .../cases 创建 case（body.user_utterances 取对应数组）。用于 baseline/candidate 实验对比。",
+  "cases": [
+    {
+      "title": "implicit_confirm_hometown",
+      "user_utterances": [
+        "你好。",
+        "我生于1962年，长在湖南长沙，小时候就在那边读的小学。",
+        "嗯，是的。",
+        "后来六十年代末家里也没搬，一直在那边。"
+      ]
+    },
+    {
+      "title": "emotional_long_memory",
+      "user_utterances": [
+        "今天想说说我们村那棵老槐树。",
+        "日本鬼子在的时候，为了望风把高过墙头的树都砍了，村里人硬是想办法保住学校里那棵，怕得很又舍不得。",
+        "我现在一说起来心里还堵得慌，那时候老人们夜里都不敢大声说话。"
+      ]
+    },
+    {
+      "title": "childhood_rich_no_job_yet",
+      "user_utterances": [
+        "我生在密云的一个村里，河边小时候常去玩。",
+        "那条河老早就改过道，我们村边上那段现在叫沙河了，明代就改过。",
+        "小学是庙改的，院子里有棵楸树，我们班男生都抱不过来。",
+        "操场北边是沙坨子，我们小孩爱往上爬，一脚深一脚浅的。"
+      ]
+    },
+    {
+      "title": "interview_brief_ack",
+      "user_utterances": [
+        "我那年冬天第一次见李老师，她借我一件棉袄穿。",
+        "嗯对，就是那件蓝布的。",
+        "后来她调到县城了，我再也没见过她。"
+      ]
+    }
+  ]
+}