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/ui-conversation/src/client/chat/MessageIconActions.tsx

@@ -1,4 +1,4 @@
-// Shared IconActions chrome for user, steering, and assistant messages: copy
+// Shared IconActions chrome for user and assistant messages: copy
 // live, optional branch wiring, and an optional date-aware clock.
 
 import { useCallback } from 'react'

packages/client/ui-conversation/src/client/chat/MessageItem.tsx

@@ -1,4 +1,4 @@
-// MessageItem: simple chat nodes — user and consumed-steering bubbles
+// MessageItem: simple chat nodes — user bubbles
 // (right-aligned, with clock + copy / branch IconActions), pending steering
 // (copy only), context injection, compaction marker, retry disclosure, and
 // unknown-surface JSON rows.
@@ -6,7 +6,7 @@
 import { memo, useEffect, useMemo, useState } from 'react'
 import type { ReactNode } from 'react'
 import type {
-  CompactionSummaryNode, ContextMessageNode, ModelRetryNode, SteeringMessageNode,
+  CompactionSummaryNode, ContextMessageNode, ModelRetryNode,
   TurnErrorNode, UnknownSurfaceNode, UserMessageNode,
 } from '@deepseek-ai/dsh-client-runtime/client'
 import { JsonBlock, MessageText, StateDot } from '@deepseek-ai/dsh-client-ui-primitives'
@@ -19,7 +19,6 @@ import css from './MessageItem.module.css'
 export interface MessageItemProps {
   node:
     | UserMessageNode
-    | SteeringMessageNode
     | ContextMessageNode
     | CompactionSummaryNode
     | ModelRetryNode
@@ -225,7 +224,6 @@ export const MessageItem = memo(function MessageItem({
   const truncated = (total: number): string => t('json.truncated', { total })
   switch (node.kind) {
     case 'user':
-    case 'steering':
       return (
         <UserStyleBubble
           content={node.content}