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/workflow/workflow-workerthread/tests/workflow-workerthread.spec.ts

@@ -83,6 +83,7 @@ class StubProvider implements SubagentProvider {
     }
     return {
       id: AgentId(`stub-child-${index}`),
+      started: Promise.resolve(),
       result,
       cancel: (reason?: string) => {
         controlled.cancelled = reason ?? 'cancelled'
@@ -222,6 +223,7 @@ describe('dsh-workflow-workerthread', () => {
         inheritsParentContext: false,
         start: () => ({
           id: AgentId('reject-child'),
+          started: Promise.resolve(),
           result: Promise.reject(new Error('backend exploded')),
           cancel: () => { /* nothing in flight */ },
           dispose: () => Promise.resolve(),
@@ -245,6 +247,7 @@ describe('dsh-workflow-workerthread', () => {
         inheritsParentContext: false,
         start: () => ({
           id: AgentId('bad-dispose-child'),
+          started: Promise.resolve(),
           result: Promise.resolve({ output: [{ type: 'text', text: 'fine' }], stopReason: 'completed' }),
           cancel: () => { /* settled already */ },
           dispose: () => Promise.reject(new Error('dispose exploded')),
@@ -266,6 +269,7 @@ describe('dsh-workflow-workerthread', () => {
         inheritsParentContext: false,
         start: () => ({
           id: AgentId('trap-child'),
+          started: Promise.resolve(),
           result: Promise.resolve({ output: [{ type: 'text', text: 'fine' }], stopReason: 'completed' }),
           cancel: () => { /* settled already */ },
           // The rejection VALUE's own coercion throws: a warn built with bare
@@ -530,6 +534,7 @@ describe('dsh-workflow-workerthread', () => {
           }, { once: true })
           return {
             id: AgentId('signal-only-child'),
+            started: Promise.resolve(),
             result,
             // The seam leaves a provider free to honor EITHER cancel channel;
             // this one deliberately ignores run.cancel() — only the request
@@ -572,6 +577,7 @@ describe('dsh-workflow-workerthread', () => {
           starts += 1
           return {
             id: AgentId('cancel-only-child'),
+            started: Promise.resolve(),
             result: new Promise(() => { /* only cancel() ends this child */ }),
             // Deliberately ignores the request signal — the seam leaves a
             // provider free to honor ONLY the explicit cancel() channel.
@@ -749,6 +755,7 @@ describe('dsh-workflow-workerthread', () => {
         inheritsParentContext: false,
         start: () => ({
           id: AgentId('doomed-child'),
+          started: Promise.resolve(),
           result: new Promise(() => { /* never settles; the reap is the teardown */ }),
           cancel: (reason?: string) => { cancelled.push(reason ?? 'cancelled') },
           dispose: () => Promise.reject(new Error('dispose exploded during reap')),