refactor(agent-loop): simplify observable state machine

packages/examples/acp-demo/tests/acp-agent.spec.ts

@@ -1,4 +1,5 @@
 import { describe, expect, it } from 'vitest'
+import { randomUUID } from 'node:crypto'
 import { mkdtemp } from 'node:fs/promises'
 import { join } from 'node:path'
 import { tmpdir } from 'node:os'
@@ -46,12 +47,9 @@ async function isolatedSkillsConfig(catalogDescriptionMaxLength?: number): Promi
 }
 
 async function composePrefix(ctx: Context): Promise<Message[]> {
-  const agent = { session: { header: { cwd: '/tmp' } } } as unknown as Agent
-  const empty: Message[] = []
-  return await agentEvents(ctx, agent).waterfall(
-    'agent/session-prefix', empty, new AbortController().signal,
-    () => Promise.resolve(empty),
-  )
+  const agent = ctx.agentLoop.create(SessionId(`acp-demo-prefix-${randomUUID()}`), {}, { cwd: '/tmp' })
+  await agentEvents(ctx, agent).serial('agent/step', 1, 1, new AbortController().signal)
+  return agent.session.deriveMessages()
 }
 
 async function withIsolatedSkillHomes<T>(run: () => Promise<T>): Promise<T> {