fix(dispatch): 孤儿恢复重建失败回滚认领，消除任务滞留 dispatched 静默丢失

[DeliciousBuding]

缺陷（#2154 Ampere P2-1）

ClaimOrphanedTasks 先将任务 queued→dispatched 独立提交，随后 redeliverOrphanedTask 的三处上下文重建失败（task / trigger message / agent-instance 加载）裸 return，无状态回滚。任务遂滞留 dispatched 且无 outbox 行：

• claim 谓词只认 status='queued' → sweeper 永不重新认领
• outbox sweeper 无行可扫
• 只能等 TTL scanner 超时 → 工作被静默丢失

修复

• repository.RequeueClaimedOrphanTask：条件 CAS 回滚 dispatched→queued；已流转（running/done/failed）任务绝不被误改。
• redeliverOrphanedTask 三处失败路径补 rollbackOrphanClaim（日志 + 计数）。
• 新增指标 agenthub_dispatch_orphan_claim_rolled_back_total。

证据

• L0：单测 4 项，含缺陷回归（重建失败→回滚 queued→下轮可重新认领）；make test 全绿（MAKE_TEST_EXIT=0，无 FAIL 行）
• L1：真 PG16 集成 5/5（含新增回滚 + 防误改 2 项）
• 6 项 verifier（doc-ssot / ci-gates / i18n-deadkeys / openapi-contract / migration-idempotency / i18n-terminology）+ git diff --check 全过
• migrations 数不变（72）

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