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/src/surface.ts

@@ -15,13 +15,12 @@ const SURFACE_EVENT_TYPES = new Set<string>([
   'user/message',
   'assistant/message',
   'tool/result',
-  'steering/message',
 ])
 
 /**
  * Whether an event type can join the model-visible surface.
  * @param type - event type to test.
- * @returns true for one of the four message-producing event types.
+ * @returns true for one of the three message-producing event types.
  */
 export function isSurfaceEligibleType(type: string): boolean {
   return SURFACE_EVENT_TYPES.has(type)