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_knowledge_mysql8.sql

@@ -329,6 +329,7 @@ CREATE TABLE IF NOT EXISTS `aihr_knowledge_conversation` (
   `context_json` json DEFAULT NULL COMMENT '最近六轮脱敏短期上下文',
   `state_version` bigint NOT NULL DEFAULT 0 COMMENT '受约束Agent结构化状态乐观版本',
   `state_json` json DEFAULT NULL COMMENT '受控引用元数据和结构化槽位；不保存工具事实正文',
+  `pending_clarification_json` varchar(1000) DEFAULT NULL COMMENT '挂起的澄清',
   `expires_time` datetime NOT NULL COMMENT '不活跃过期时间',
   `create_time` datetime NOT NULL COMMENT '创建时间',
   `update_time` datetime NOT NULL COMMENT '更新时间',