feat: PromptProvider 化 + $DEFLECTION_POOL 单一事实源注入 + runtime 必填化

- PromptProvider(app/engine/services/ai_service.py): 租户提示词四件套
  from_dir 加载;build_system 把 system.md 的 $DEFLECTION_POOL 占位符替换为
  wall.json deflections 话术池(按语言分组一行,EN/ZH/ES)再拼事实卡;
  extractor/evaluator 的 $existing_info/$messages/$profile/$recent_messages
  替换逻辑自 AIService 原样搬入(行为等价);删模块级 import 时读文件的
  PROMPTS_DIR 与四个常量,保留 DEFAULT_PROVIDER(jiancai 出厂,__file__ 锚定)
- 两租户 system.md「三类请求的处理」第2条的迂回话术枚举 → $DEFLECTION_POOL
  占位符(保留段落说明文字),话术只存 wall.json 一份,改一处两边同步变
- TenantRuntime 加 prompts 字段(加载期 from_dir(seed_dir/prompts));
  ConversationManager 的 runtime 转必填(keyword-only),默认 AIService 用
  runtime.prompts + compiled_wall.deflections 组装
- app/engine/prompts/ 删除(grep 无代码消费方,jiancai 出厂由
  seed_profiles/jiancai/prompts 承接);test_company_facts/
  test_system_prompt_dual_contact 的 cwd 相对路径改 Path(__file__) 锚定,
  test_extractor_evaluator_prompts 断言对象改 DEFAULT_PROVIDER(语义等价)
- 新增 tests/test_prompt_provider.py(14 用例): 加载/注入/格式/占位符防漂移/
  extractor-evaluator 组装/registry 接线/runtime 必填/池线过两墙

Co-Authored-By: Claude Code <noreply@anthropic.com>

tests/test_lead_email_trigger.py

@@ -10,6 +10,7 @@ from app.engine.models import (
 )
 from app.engine.services.conversation import CONTACT_ASK_TRIGGER_RE, ConversationManager
 from app.engine.services import email_service
+from tests.conftest import build_jiancai_runtime
 
 
 class FakeAI:
@@ -77,7 +78,9 @@ def _setup(async_db, monkeypatch, source=ConversationSource.widget, ai=None):
 
     async def run():
         async with async_db() as db:
-            manager = ConversationManager(db, ai_service=ai or FakeAI())
+            manager = ConversationManager(
+                db, runtime=build_jiancai_runtime(),
+                ai_service=ai or FakeAI())
             conv = await manager.create_conversation(source=source)
             return manager, conv
     return asyncio.run(run()), sent