feat(daemon): Mail 失败通知 v2.0 — api_error retry + 通知增强

P1: api_error rate_limit/500/503 改为可恢复 retry（should_retry=True，60s cooldown）
P2: 通知模板动态化（reason 人话翻译 + detail 信息 + 重试情况 + AI Native 知识库）

设计文档：§20.7 (20-task-type-architecture.md)

src/daemon/spawner.py

@@ -845,6 +845,8 @@ curl -X POST http://{api_host}:{api_port}/api/projects/{project_id}/tasks/{task_
                 cls.get("retry_field", "retry_count")
             )
         elif outcome == "api_error":
+            # A9: [DEPRECATED] api_error 已改为 should_retry=True 走续杯路径。
+            # 此分支理论上不再命中，保留作为安全兜底。
             # A9: 429/API 错误 → release counter(on_complete)+ 推回 pending + 冷却
             # 有上限:api_retry_count 累计达 max_retries 则标 failed
             await self._do_on_complete_async(on_complete, agent_id, outcome)
@@ -1842,7 +1844,8 @@ curl -X POST http://{api_host}:{api_port}/api/projects/{project_id}/tasks/{task_
                         "retry_field": "retry_count", "cooldown_seconds": 60}
             if any(kw in stderr_lower for kw in [
                    "rate_limit", "500", "503", "api error"]):
-                return {"outcome": "api_error", "should_retry": False}
+                return {"outcome": "api_error", "should_retry": True,
+                        "retry_field": "retry_count", "cooldown_seconds": 60}
             if any(kw in stderr_lower for kw in [
                    "compaction-diag", "context-overflow"]):
                 return {"outcome": "compact_failed", "should_retry": False}