feat(subagent): sibling delegations are concurrency-safe

One assistant message carrying several subagent calls now overlaps them
under the rolling pool (maxParallelToolCalls) instead of serializing each
foreground delegation behind an exclusive barrier; results still commit
in model order.

tool-subagent declares isConcurrencySafe: () => true for every call form:
children work in their own sessions, a run never mutates the parent
session, and sibling workspace coordination already belongs to the model
for background, continuable, and workflow children. The former
serial-execution pin flips to a parallel pin plus a gated genuine-overlap
test, and the authored subagent-parallel snapshot pins the assembled
transcript (tool/call, tool/call, tool/result, tool/result) with
interchangeable twin children, which replay race-free under the existing
first-call binding and createdAt harvest ordering.

The provider seam now requires concurrent starts and continuable
preparations for distinct children to isolate operation-local state,
cancellation, settlement, and cleanup; a continuable gate test proves a
cancelled preparation leaves no Agent or durable Session while its
sibling persists independently.

Closes deepseek-harness/deepseek-harness#1489

.agents/notes/implemented/feature/2026-07-10-parallel-tool-call-execution.zh.md

@@ -60,7 +60,7 @@ Code Mode 仍不使用此调度器，因为模型只会发出一个原生 `run_c
 
 `maxParallelToolCalls` 是 AgentLoop 的正整数部署上限，由工厂创建的所有 agent（智能体）共享。默认值为 `10`；`1` 保持串行执行。字段和默认值的精确定义见生成的[配置目录](../../../../docs/config-catalog.md)。
 
-当前实现中的声明保持保守。Web 搜索、Web 获取和文件系统读取选择并行。文件系统写入与编辑、bash 工具、subagent 委派、工作流、用户交互、todo 变更、Code Mode 以及 Cordis 变更工具仍按独占方式执行。subagent 可能共享父级的工作区或外部资源，而一元分类器无法证明并列委派的作用互不重叠。Bash 没有已证明的输入敏感分类器，因此仍按独占方式执行。
+当前实现中的声明保持保守。Web 搜索、Web 获取、文件系统读取、会话查询的 trace/read 工具和 subagent 委派选择并行；委派之所以并行，是因为子 agent 在自己的会话中工作，其运行绝不变更父会话，并列委派间的工作区协调由模型负责（[并行 subagent Agent Note](2026-08-09-parallel-subagent-delegations.md)）。文件系统写入与编辑、bash 工具、会话查询的 search 工具、工作流、用户交互、todo 变更、Code Mode 以及 Cordis 变更工具仍按独占方式执行。Bash 没有已证明的输入敏感分类器，因此仍按独占方式执行。
 
 文件系统读取依赖一个范围很窄的记录器例外：其同步观察更新可以不按顺序结算，但写入和编辑在变更前会重新检查已观察的版本，因此陈旧状态只会导致 `FS_STALE_VERSION`。