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.

.agents/notes/implemented/simplification/2026-07-31-web-ui-no-steer-entry-or-interjection-chrome.md

@@ -30,9 +30,11 @@ Keep host and runtime steering intact. Remove only the Web UI entry and chrome:
 
 - Web users cannot steer from the composer or `ctx.conversation.send`; stop/cancel and Queue remain the only mid-turn controls.
 - Host-wire and non-Web clients can still steer; the Web client shows those messages without labeling them as interjections.
+- Non-user next-step items (`agent.inject` context: approval notices, task completion, attached snapshots) broadcast with the `context` placement and never render as pending steering bubbles; they stay invisible until claimed as durable `user/message` context cards.
 - Reintroducing a dedicated steer UI would need a new product decision; do not revive the mode union or badge without one.
 
 ## Testing
 
 - `packages/client/ui-conversation` unit/jsdom coverage: input machine enter/sink, ConversationService routing, MessageItem steering arm (no 「插话」), InputBar submit.
 - `apps/web/tests/steering.e2e.ts` keyless replay plus updated `settled.expected.md` (steer text without badge).
+- `packages/host/apiproxy` `session/queue` projection test asserts user-origin next-step items stay `steering` while plugin-origin items land as `context`.

.agents/notes/implemented/simplification/2026-07-31-web-ui-no-steer-entry-or-interjection-chrome.zh.md

@@ -30,9 +30,11 @@ Status: implemented
 
 - Web 用户无法从 composer 或 `ctx.conversation.send` steer；中途控制只剩停止／取消与 Queue。
 - Host 线缆与非 Web 客户端仍可 steer；Web 客户端展示这些消息时不再标成插话。
+- 非用户来源的 next-step 项（`agent.inject` 上下文：审批通知、任务完成、附加快照）以 `context` placement 广播，绝不渲染为待处理 steering 气泡；领取为持久 `user/message` context card 前保持不可见。
 - 若要重新引入专用 steer UI，需要新的产品决策；没有决策就不要复活 mode 联合类型或徽章。
 
 ## 测试
 
 - `packages/client/ui-conversation` unit／jsdom 覆盖：input machine enter／sink、ConversationService 路由、MessageItem steering 分支（无「插话」）、InputBar submit。
 - `apps/web/tests/steering.e2e.ts` 无密钥回放，以及更新后的 `settled.expected.md`（有 steer 正文、无徽章）。
+- `packages/host/apiproxy` 的 `session/queue` 投影测试断言用户来源的 next-step 项保持 `steering`，而插件来源的项落入 `context`。