refactor(agent-loop): drop the steering/message session event

Steer, inject, and followup now land as durable user/message events on the
session surface; the steering/message event type and its ConversationNode
kind are removed from the client projection. Update tests, docs, generated
catalogs, and agent notes to match, and align the steering e2e fixture and
prompt inventory assertions with the durable user/message landing.

packages/client/connection/tests/fixture.spec.ts

@@ -301,7 +301,7 @@ describe('createFixtureApi', () => {
     expect(idleCancel.result).toMatchObject({ ok: true })
   })
 
-  it('steer during a replay inserts a steering message and the replay continues to completion', async () => {
+  it('steer during a replay lands a user/message inside the current turn and the replay continues', async () => {
     const api = createFixtureApi()
     const created = await api.sessions.create(req({}))
     if (!created.result.ok) throw new Error('create failed')
@@ -314,7 +314,7 @@ describe('createFixtureApi', () => {
     await api.sessions.prompt(req({ sessionId: id, mode: 'steer' as const, content: [{ type: 'text' as const, text: '插话' }] }))
     const frames = await framesPromise
     const types = frames.filter((f): f is Extract<MuxFrame, { type: 'session/event' }> => f.type === 'session/event').map(f => f.event.type)
-    expect(types).toContain('steering/message')
+    expect(JSON.stringify(frames)).toContain('插话')
     expect(types.at(-1)).toBe('turn/end') // steer did not restart the turn
   })
 
@@ -362,7 +362,7 @@ describe('createFixtureApi', () => {
     }))
     const frames = await framesPromise
     const types = frames.filter((f): f is Extract<MuxFrame, { type: 'session/event' }> => f.type === 'session/event').map(f => f.event.type)
-    expect(types[0]).toBe('turn/start') // idle steer degraded to a queued turn, not a steering insert
+    expect(types[0]).toBe('turn/start') // idle steer degraded to a queued turn, not an in-turn insert
   })
 
   it('gamma interval flip emits host/session-status and a running log-less session subscribes at lastSeq -1', async () => {