feat(web): choose the agent preset on the new-session screen

The composer seat spent nearly all its life disabled: a session's
composition is fixed once a turn has run. Move the choice to the
new-session screen beside the workspace picker, where it still works,
and let the session header report what a running session runs.

The hero pick is staged rather than applied — that screen precedes the
session it belongs to. It lands when a session becomes current and is
still blank, which covers both the session a workspace connect creates
and the blank one it reuses; riding `sessions.create` would miss the
second. It is spent on first use, matching the workspace picker.

Fix the durability the header field claimed but never had: `agentPreset`
was declared on `SessionHeader` and dropped by the JSONL header line, the
SQLite sessions row, the derived query index, and the cold list
projection, so every resumed session came back composed from nothing.

Add the web e2e lane that would have caught it — the one lane that mounts
the shipped roster, which needed `cordis:group` in the scaffold's Loader
builtins, as `mountRootInclude` already registers.

packages/client/README.zh.md

@@ -32,6 +32,7 @@ dsh web GUI 的浏览器侧：shell 启动、浏览器与宿主通信、共享 U
 | [`ui-permission/`](ui-permission/README.md) | 配置默认权限并切换当前会话的访问模式。 |
 | [`ui-plan/`](ui-plan/README.md) | 展示生效中的 plan mode 状态及其退出控件。 |
 | [`ui-question/`](ui-question/README.md) | 展示 agent 请求的交互式问题。 |
+| [`ui-agent-preset/`](ui-agent-preset/README.md) | 选择会话的 agent 预设，并创作预设组装。 |
 | [`ui-settings/`](ui-settings/README.md) | 承载设置界面及其扩展区域。 |
 | [`ui-settings-general/`](ui-settings-general/README.md) | 提供常规设置分区。 |
 | [`ui-models/`](ui-models/README.md) | 提供模型提供方配置与 DeepSeek 配置引导。 |