fix(scope): close final ownership races

Drain idle injection flushes before agent teardown, snapshot approval and subagent provider inputs, and gate subagent lifecycle events on real child readiness. Align the RFCs and generated contracts with the hardened behavior.

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

@@ -289,11 +289,23 @@ export function startInProcessRun(
     return created.agent
   })()
 
+  // Provider readiness is a distinct lifecycle boundary from accepting the
+  // request. It resolves only after the factory has published the child and
+  // returned its handle, so SubagentService can emit `subagent/start` while
+  // `ctx.agents.get(childId)` is guaranteed to resolve. The result path awaits
+  // THIS SAME promise immediately, which also observes a readiness rejection
+  // when the driver is invoked directly rather than through SubagentService.
+  const started: Promise<void> = creation.then(() => undefined)
+
   const result: Promise<SubagentResult> = (async () => {
     try {
       let liveChild: Agent
       try {
-        liveChild = await creation
+        await started
+        // `creation` assigns `child` before it fulfills, and `started` is its
+        // direct fulfillment projection. The cast records that local invariant
+        // without manufacturing an unreachable runtime branch.
+        liveChild = child as Agent
       } catch (error: unknown) {
         if (isManualDisposeRequested()) return { output: [], stopReason: 'aborted' }
         throw error instanceof Error ? error : new Error('subagent child creation failed with a non-Error value', { cause: error })
@@ -313,6 +325,7 @@ export function startInProcessRun(
   let disposing: Promise<void> | undefined
   return {
     id: childId,
+    started,
     result,
     cancel(reason?: string): void {
       requestCancel(reason ?? 'subagent cancelled')