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/tests/scripted-provider.ts

@@ -33,6 +33,8 @@ export interface Config {
   inheritsParentContext?: boolean
   /** Structured value returned when the request asks for one. */
   structured?: unknown
+  /** Observes each start; the child's result additionally waits for the returned promise. */
+  onStart?: (request: SubagentStartRequest) => Promise<void> | void
 }
 
 /** Scripted provider whose result aborts if its signal or disposer wins first. */
@@ -68,9 +70,10 @@ class ScriptedSubagentProvider implements SubagentProvider {
       ...wantsStructured ? { structured: this.config.structured ?? { reply } } : {},
       stopReason: state.cancelled ? 'aborted' : stopReason,
     })
-    const result = new Promise<SubagentResult>((resolve) => {
+    const gate = Promise.resolve(this.config.onStart?.(request))
+    const result = gate.then(() => new Promise<SubagentResult>((resolve) => {
       setTimeout(() => { resolve(resultFor()) }, 0)
-    }).finally(() => {
+    })).finally(() => {
       request.signal.removeEventListener('abort', onAbort)
     })