feat(aihr): add evidence grading, clarification loop and route exits to ask agent

- narrow amount-semantics clarification to adjudicative intents only
- persist pending clarification on conversation and resolve follow-up
  replies as ANSWERED/GAVE_UP/NEW_TOPIC (max one round per field)
- add AihrEvidenceGradingService: FULL/PARTIAL/GUIDANCE/ROUTE arbitration,
  policy-claim post-validation, reference answers labeled as non-formal
- unblock retrieval source gating; tag citations with evidence tier,
  keep PUBLISHED/HUMAN_VERIFIED/effective-date authorization gates
- replace operator-facing no-evidence copy with actionable route exits
  (human help, direct finance channel, web research)
- allow fragment-less down feedback for own NO_EVIDENCE queries so
  ROUTE answers can escalate to human help (fragment_id nullable)
- migrations: aihr_20260804_pending_clarification_mysql8.sql,
  aihr_20260804_feedback_fragment_nullable_mysql8.sql

backend/script/sql/aihr_practice_mysql8.sql

@@ -329,7 +329,7 @@ CREATE TABLE IF NOT EXISTS `aihr_knowledge_answer_feedback` (
   `tenant_id` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT '000000' COMMENT '租户编号',
   `query_text` varchar(1000) NOT NULL COMMENT '问题原文',
   `query_norm` varchar(500) NOT NULL COMMENT '规范化问题',
-  `fragment_id` bigint NOT NULL COMMENT '知识片段ID',
+  `fragment_id` bigint DEFAULT NULL COMMENT '知识片段ID；无依据回答的反馈为空',
   `verdict` varchar(10) NOT NULL COMMENT '反馈: up/down',
   `category` varchar(100) DEFAULT NULL COMMENT '知识分类',
   `position` varchar(80) DEFAULT NULL COMMENT '岗位',