refactor(agent): complete inbox lifecycle migration

examples/acp-agent/tests/fixtures/subagent-durability-failure.ts

@@ -84,12 +84,16 @@ export function apply(ctx: Context): void {
   // runs, so the queued FIFO order is what the transcript records. The first
   // child enqueue is the initial delegation, which also pins the real child id.
   let accepted = 0
-  ctx.on('agent/inbox/enqueue', (agent) => {
+  ctx.on('agent/inbox/inserted', (agent) => {
     if (agent.session.header.parentSession === undefined) return
     if (realChildId === undefined) realChildId = agent.session.header.id
     accepted += 1
     if (accepted >= 3) followupsAccepted.resolve(undefined)
   })
+  ctx.on('agent/pre-step', async (agent, _messages, _context, next) => {
+    if (agent.session.header.parentSession !== undefined) await followupsAccepted.promise
+    return next()
+  })
 
   // The child's ordinary per-turn flushes succeed; only the final continuation
   // turn's durability checkpoint fails, turning that turn/end into a durable