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

packages/subagent/tool-subagent/src/index.ts

@@ -330,6 +330,9 @@ export function apply(ctx: Context, config: Config): void {
               : outputValueText(value.output),
         }],
       },
+      // Children never mutate the parent session; the one parent-owned write
+      // (tasks.start) is a synchronous commutative insertion.
+      isConcurrencySafe: () => true,
       async execute(args, exec) {
         const parent = exec.agent
         if (!parent) {