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/core/session/tests/surface.spec.ts

@@ -711,18 +711,15 @@ describe('deriveMessages with surface', () => {
     expect(messages[0]!.content[0]).toMatchObject({ type: 'text', text: 'compacted' })
   })
 
-  it('injected-context and steering/message appear on surface', () => {
+  it('injected-context and user messages appear on surface', () => {
     const s = new Session(SessionId('ctx'))
     s.append('user/message', createUserMessage({
       content: [{ type: 'text', text: 'file changed' }], source: { kind: 'plugin', plugin: 'watcher' },
     }), { surfaceOp: 'append' })
-    s.append('steering/message', {
-      turn: 1,
-      message: createUserMessage({
-        content: [{ type: 'text', text: 'focus' }],
-        source: { kind: 'user' },
-      }),
-    }, { surfaceOp: 'append' })
+    s.append('user/message', createUserMessage({
+      content: [{ type: 'text', text: 'focus' }],
+      source: { kind: 'user' },
+    }), { surfaceOp: 'append' })
     const messages = s.deriveMessages()
     expect(messages).toHaveLength(2)
     expect(messages[0]!.content).toEqual([{ type: 'text', text: 'file changed' }])
@@ -831,7 +828,6 @@ describe('surface type guards', () => {
     expect(isSurfaceEligibleType('user/message')).toBe(true)
     expect(isSurfaceEligibleType('assistant/message')).toBe(true)
     expect(isSurfaceEligibleType('tool/result')).toBe(true)
-    expect(isSurfaceEligibleType('steering/message')).toBe(true)
     expect(isSurfaceEligibleType('turn/start')).toBe(false)
     expect(isSurfaceEligibleType('assistant/chunk')).toBe(false)
   })