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-fork/tests/subagent-fork.spec.ts

@@ -71,6 +71,23 @@ describe('completedTurnPrefix', () => {
 })
 
 describe('dsh-subagent-fork', () => {
+  it('emits subagent/start only after the seeded child is published', async () => {
+    const { ctx, parent } = await setup([textResponse('child answer')])
+    let childAtStart: ReturnType<typeof ctx.agents.get>
+    ctx.on('subagent/start', (info) => {
+      if (info.provider === 'fork') childAtStart = ctx.agents.get(info.id)
+    })
+
+    const run = ctx.subagents.start('fork', { prompt: [{ type: 'text', text: 'child q' }], parent })
+    expect(childAtStart).toBeUndefined()
+    await run.started
+    expect(childAtStart).toBe(ctx.agents.get(run.id))
+    expect(childAtStart?.id).toBe(run.id)
+
+    await run.result
+    await run.dispose()
+  })
+
   it('forks an UNSEEDED (fresh) child when the parent has no completed turn', async () => {
     // The parent has never completed a turn → empty prefix → the provider omits
     // the seed → the child runs fresh. Exercises the `seed.length > 0` false arm.