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/host/apiproxy/src/api/events.ts

@@ -36,8 +36,8 @@ export type ToolEventView =
 export interface QueuedInboxItem {
   /** Message identity used by inbox mutations. */
   id: MessageId
-  /** Agent-resolved FIFO placement; clients render queued and steering items on different surfaces. */
-  placement: 'queued' | 'steering'
+  /** Agent-resolved FIFO placement; queued and steering items render on different surfaces, context items stay invisible until claimed. */
+  placement: 'queued' | 'steering' | 'context'
   /** Complete pending message; it is not durable until the Agent claims it. */
   message: Message
 }