fix(apiproxy): stop projecting injected context as user steering

next-step items now split by origin: user-origin messages keep the
'steering' placement, while agent.inject context (approval notices, task
completion notices, attached snapshots) carries a new 'context' placement
that no surface renders until it is claimed as a durable user/message
context card. Widen the placement unions on the wire and in the client
runtime, add a projection test case, and document the split in the
apiproxy/ui-conversation READMEs and the web-steer chrome agent note.

packages/client/runtime/src/client/sessions/conversation.ts

@@ -266,7 +266,7 @@ export interface QueuedMessage {
   /** Stable message identity used for transient-to-durable steering handoff. */
   readonly messageId: MessageId
   /** Agent-resolved placement; only queued rows accept queue mutations. */
-  readonly placement: 'queued' | 'steering'
+  readonly placement: 'queued' | 'steering' | 'context'
   /** Complete content used to render pending steering before it becomes durable. */
   readonly content: readonly ContentBlock[]
   readonly preview: string