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/tests/api-proxy-commands.spec.ts

@@ -360,6 +360,14 @@ describe('session/queue frames', () => {
     ]
     agent.inbox.splice('next-turn', 0, 1, [edited])
     frames.push(await nextFrame(iterator), await nextFrame(iterator))
+    const injected = freezeMessage({
+      id: MessageId('m-3'),
+      role: 'user',
+      content: [{ type: 'text' as const, text: 'injected context' }],
+      source: { kind: 'plugin' as const, plugin: 'approval' },
+    })
+    agent.inbox.splice('next-step', 0, 0, [injected])
+    frames.push(await nextFrame(iterator), await nextFrame(iterator))
     abort.abort()
     await iterator.return?.()
 
@@ -380,6 +388,15 @@ describe('session/queue frames', () => {
           { id: steering.id, placement: 'steering', message: steering },
         ],
       },
+      {
+        type: 'session/queue',
+        sessionId: agent.id,
+        items: [
+          { id: edited.id, placement: 'queued', message: edited },
+          { id: injected.id, placement: 'context', message: injected },
+          { id: steering.id, placement: 'steering', message: steering },
+        ],
+      },
     ])
   })
 })