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-spawn/tests/subagent-spawn.spec.ts

@@ -55,6 +55,25 @@ describe('dsh-subagent-spawn', () => {
     await run.dispose()
   })
 
+  it('emits subagent/start only after the fresh 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 === 'spawn') childAtStart = ctx.agents.get(info.id)
+    })
+
+    const run = ctx.subagents.start('spawn', { prompt: [{ type: 'text', text: 'do X' }], parent })
+    // Creation is asynchronous; no lifecycle claim is made while the child is
+    // still inside its unpublished setup transaction.
+    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('gives the child its OWN session (not the parent\'s), with parentSession lineage', async () => {
     const { ctx, parent } = await setup([textResponse('hi')])
     const run = ctx.subagents.start('spawn', { prompt: [{ type: 'text', text: 'p' }], parent })