Record U3B re-attempt after host integration steps 1-3

Why: cursor attempt 1 of U3B re-dispatch (run_id unknown) = failed-unknown exit 1; even after Backend L1→main merged at 3f4b34a, RDM L1→main merged at 5a27d78, Dev modules/backend bumped to 3f4b34a (commit 48f5126), cursor-agent still produced invalid terminal record.

Evidence: receipts /tmp/iam-link-u3b-run.json run_id (latest); Backend PR #219 merged (3f4b34a); RDM PR #450 merged (5a27d78); Dev commit 48f5126.

Impact: U3B in blocked state again; subsequent session should retry with attempt 2/3 of cursor.

Refs: plan revision 3 U3; PR #628.

.task/feature-iam-role-permission-linked-capabilities-u3b-rdm-projectfailure-integration.md

@@ -75,11 +75,10 @@
   - attempt 3 (run_id=d3a1…) terminal=failed-unknown exit 1。
 - 验证：NOT_RUN（cursor 候选耗尽无成功 transport）
 - 影响与剩余项：
-  - cursor 候选已耗尽（3/3），未启 codex 紧急。
-  - 后续会话建议：
-    1. **先做 Backend L1→main 合入 + Dev `modules/backend` gitlink bump 到含 U3A 的 Backend commit**（host 集成工作，不属实施），然后 RDM 端就能消费 `CapabilityExplain`；U3B 重试时即可放开 `pins/agent-system-backend` 升级约束
-    2. 拆 U3B 为两个更小单元：U3B-A（4 类错误分类 + 不泄露，独立于 Backend） + U3B-B（消费 `CapabilityExplain`，需要 Backend main 集成）
-  - U4 仍依赖 U3 完成
+  - host 集成步骤 1-3 已完成：Backend L1→main merged at `3f4b34a`，RDM L1→main merged at `5a27d78`，Dev `modules/backend` bumped to `3f4b34a`（commit `48f5126`）
+  - Dev `modules/rdm` 不存在（Dev 无 RDM 子模块），step 4 不适用
+  - U3B cursor 重试 attempt 1（已放开 pins/agent-system-backend bump）= failed-unknown exit 1
+  - 后续会话：cursor attempt 2/3 重试 U3B 或派发 U4/U5/U6
 
 ## 合入与清理