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/tests/chat-branch-tails.spec.tsx

@@ -1,5 +1,5 @@
 // @vitest-environment jsdom
-// Remaining chat branch tails: MessageItem context/unknown/steering arms,
+// Remaining chat branch tails: MessageItem context/unknown arms,
 // user IconActions, StatsLine no-cache join,
 // AssistantMarkdown single-line reasoning. (Tool-row dispatch tails live
 // with the keyed-slot machinery specs since the tool ring dissolved into
@@ -102,30 +102,6 @@ describe('MessageItem arms', () => {
     fireEvent.click(screen.getByRole('button', { name: '复制' }))
   })
 
-  it('consumed steering renders copy and branch actions without a badge', () => {
-    const writeText = vi.fn().mockResolvedValue(undefined)
-    Object.defineProperty(navigator, 'clipboard', {
-      configurable: true,
-      value: { writeText },
-    })
-    const fork = vi.fn()
-    const view = render(
-      <MessageItem t={t} node={{
-        kind: 'steering', messageId: 'steer-message', seq: 2, time: 1_000, turn: 1, source: null,
-        content: [{ type: 'text', text: 'steer!' }, { type: 'image', data: 'x' }] as never,
-      } as never}
-      onFork={fork}
-      />,
-    )
-    expect(view.queryByText('插话')).toBeNull()
-    expect(view.getByText('steer!')).toBeTruthy()
-    expect(view.getByText(/附加内容块/)).toBeTruthy()
-    fireEvent.click(view.getByRole('button', { name: '复制' }))
-    expect(writeText).toHaveBeenCalledWith('steer!')
-    fireEvent.click(view.getByRole('button', { name: '在新对话中分支' }))
-    expect(fork).toHaveBeenCalledWith(2)
-  })
-
   it('context uses the Tool calls disclosure chrome and keeps its JSON collapsed by default', () => {
     const ctxView = render(
       <MessageItem t={t} node={{

packages/client/ui-conversation/tests/chat-view.spec.tsx

@@ -292,8 +292,7 @@ describe('ChatView', () => {
         nodes: [
           assistant(1, 'working'),
           {
-            kind: 'steering', messageId: pending.messageId,
-            seq: 2, time: 2_000, turn: 1,
+            kind: 'user', seq: 2, time: 2_000,
             content: [{ type: 'text', text: 'interrupt now' }], source: null,
           },
         ],
@@ -320,8 +319,7 @@ describe('ChatView', () => {
     const h = makeHarness({
       queue: [pending],
       nodes: [{
-        kind: 'steering', messageId: pending.messageId,
-        seq: 2, time: 2_000, turn: 1,
+        kind: 'user', seq: 2, time: 2_000,
         content: pending.content, source: null,
       }],
       running: true,