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/runtime/src/client/index.ts

@@ -48,7 +48,7 @@ export type {
   AssistantTiming, CodeSubCall, CommandNode, CompactionSummaryNode, ComposerPhase,
   ContextMessageNode, ConversationNode, ConversationSnapshot, ModelRetryNode, QueuedMessage,
   RunningToolCall,
-  SteeringMessageNode, TodoItem, ToolResultNode, TurnErrorNode, UnknownSurfaceNode, UserMessageNode,
+  TodoItem, ToolResultNode, TurnErrorNode, UnknownSurfaceNode, UserMessageNode,
 } from './sessions/conversation.ts'
 export type {
   ConversationContext, ConversationContextOriginKind,

packages/client/runtime/src/client/session-history/history-fold.ts

@@ -144,12 +144,6 @@ function materializeNode(
         ...(requestConfig === undefined ? {} : { requestConfig }),
         ...(assistantTiming === undefined ? {} : { timing: assistantTiming }),
       }
-    case 'steering/message':
-      return {
-        kind: 'steering', messageId: event.data.message.id,
-        seq: event.seq, time: event.time, turn: event.data.turn,
-        content: event.data.message.content, source: event.data.message.source,
-      }
     case 'tool/result': {
       const result = event.data.message.content[0]
       const callId = String(event.data.message.source.callId)

packages/client/runtime/src/client/sessions/conversation.ts

@@ -102,19 +102,6 @@ export interface AssistantMessageNode {
   interrupted?: true
 }
 
-/** A steering message injected mid-turn. */
-export interface SteeringMessageNode {
-  kind: 'steering'
-  /** Stable identity shared with its pre-admission inbox occurrence. */
-  messageId: MessageId
-  seq: number
-  /** Unix epoch ms from the source session event. */
-  time: number
-  turn: number
-  content: readonly ContentBlock[]
-  source: unknown
-}
-
 /** A context/system injection surfaced in the flow. */
 export interface ContextMessageNode {
   kind: 'context'
@@ -236,7 +223,6 @@ export interface CommandNode {
 export type ConversationNode =
   | UserMessageNode
   | AssistantMessageNode
-  | SteeringMessageNode
   | ContextMessageNode
   | ModelRetryNode
   | TurnErrorNode

packages/client/runtime/src/client/sessions/session.ts

@@ -656,12 +656,8 @@ export class Session implements SessionFace {
 
   /** Retire the first matching live steering occurrence when its durable message takes over. */
   private handoffPendingSteering(event: SessionEvent): void {
-    const message = event.type === 'user/message'
-      ? event.data
-      : event.type === 'steering/message'
-        ? event.data.message
-        : undefined
-    if (message === undefined) return
+    if (event.type !== 'user/message') return
+    const message = event.data
     const index = this.queued.findIndex(item =>
       item.placement === 'steering' && item.messageId === message.id)
     if (index === -1) return

packages/client/runtime/src/client/sessions/transcript-adapter.ts

@@ -72,12 +72,6 @@ function materializeNode(
         turn: event.data.turn, step: event.data.step,
         blocks: toAssistantBlocks(event.data.message.content), usage: event.data.usage,
       }
-    case 'steering/message':
-      return {
-        kind: 'steering', messageId: event.data.message.id,
-        seq: event.seq, time: event.time, turn: event.data.turn,
-        content: event.data.message.content, source: event.data.message.source,
-      }
     case 'tool/result': {
       const result = event.data.message.content[0]
       const callId = String(event.data.message.source.callId)