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.

examples/acp-agent/tests/acp.snapshot.ts

@@ -418,8 +418,7 @@ it('packed ACP fixture retains every chunk row kind without changing the logical
     }
     if (cloned.type === 'user/message') delete cloned.data?.id
     if (cloned.type === 'assistant/message'
-      || cloned.type === 'tool/result'
-      || cloned.type === 'steering/message') {
+      || cloned.type === 'tool/result') {
       delete cloned.data?.message?.id
     }
     if (cloned.type === 'hook/result') delete cloned.data?.durationMs