feat(session)!: embed assistant streams in format v2

packages/bundle/headless/src/index.ts

@@ -90,8 +90,8 @@ function summarize(session: Session, firstSeq: SessionLogOffset): RunOutcome {
 
 /**
  * Project provider-reported reasoning from one owned run to stderr as it is
- * appended, while keeping final outcome derivation on the durable log.
- * @param ctx - plugin context carrying the Session event feed.
+ * streamed, while keeping final outcome derivation on the durable log.
+ * @param ctx - plugin context carrying the live Assistant frame feed.
  * @param agent - the exact Agent whose reasoning belongs to this invocation.
  * @param stderr - progress output sink.
  * @returns a disposer that also terminates an unterminated reasoning line.
@@ -101,7 +101,6 @@ function streamReasoning(
   agent: Agent,
   stderr: HeadlessIo['stderr'],
 ): () => void {
-  let started = false
   let open = false
   let endsWithNewline = true
   const close = (): void => {
@@ -110,15 +109,17 @@ function streamReasoning(
     open = false
     endsWithNewline = true
   }
-  const dispose = ctx.on('session/event', (session, event) => {
-    if (session !== agent.session) return
-    if (event.type === 'turn/start') {
+  const dispose = ctx.on('agent/assistant-stream', ({ agent: subject, frame }) => {
+    if (subject !== agent) return
+    if (frame.type === 'start') {
       close()
-      started = true
       return
     }
-    if (!started || event.type !== 'assistant/chunk') return
-    const chunk = event.data.chunk
+    if (frame.type === 'end') {
+      close()
+      return
+    }
+    const chunk = frame.chunk
     switch (chunk.type) {
       case 'reasoning-delta':
         if (chunk.text === '') return