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.

.agents/notes/implemented/architecture/2026-08-03-per-session-agent-presets.md

@@ -63,6 +63,10 @@ Which preset an unnamed session gets is a user setting (`agent-presets.default`)
 
 **The preset id is model-visible and must be logged.** It determines the tool set and prompt, so a resumed session has to restore the same composition; recording it is a session fact, not runtime state. It rides the session header beside `cwd`, and the summary carries it so a picker shows what a session actually runs rather than the deployment's current default.
 
+**A durable header field is not durable until every backend writes it.** `agentPreset` landed on `SessionHeader` with the right rationale and neither persistence backend carried it: the JSONL header line, the SQLite `sessions` row, and the derived query index each map the header column by column, so a resumed session came back with no preset and the surfaces that name it fell silent. `summarizeCold` had the same shape — it hand-built the cold list row instead of reusing the shared projection. A field declared durable needs a test that crosses a real store, not only the type that declares it.
+
+**The choice belongs to the screen where it still works.** The composer seat spent almost its whole life disabled, since the preset is fixed once a turn has run. It moved to the new-session screen beside the workspace picker, where the pick is *staged*: that screen precedes the session it applies to, and the stage lands when a session becomes current and is still blank — covering both the session a workspace connect creates and the blank one it reuses, which riding `sessions.create` would miss. It is spent on first use, matching the workspace picker beside it. What a running session runs is then a read-only label in its header: a control there would promise a switch the host refuses outright.
+
 ## Alternatives considered
 
 **Add a preset tier to the scoped registries.** `ScopedLayers.merge()` combines the global layer with exactly one exact-scope layer. A middle tier would let many sessions share one mounted composition, but it changes `dsh-scope` and every scope-aware registry to save a cost measured in milliseconds, and it gives a preset's registrations a lifetime no agent owns.

.agents/notes/implemented/architecture/2026-08-03-per-session-agent-presets.zh.md

@@ -64,6 +64,10 @@ Status: implemented
 
 **preset id 对模型可见，必须写入日志。** 它决定工具集与提示词，因此被恢复的会话必须还原同一份组装；记录它属于会话事实，而非运行时状态。它与 `cwd` 并列写在会话头部，并由会话摘要携带，使选择器显示的是某个会话实际运行的 preset，而非部署当前的默认值。
 
+**持久化的头部字段，在每个后端都写入之前都算不上持久。** `agentPreset` 带着正确的理由落在了 `SessionHeader` 上，而两个持久化后端都没有携带它：JSONL 头部行、SQLite `sessions` 行、以及派生的查询索引各自逐列映射头部，于是被恢复的会话回来时没有 preset，所有据以命名它的表层随之失声。`summarizeCold` 是同一个形状——它手工拼装冷列表行，而没有复用共享的投影。声明为持久的字段，需要一个跨越真实存储的测试，而不只是声明它的那个类型。
+
+**这个选择属于它仍然可用的那个界面。** composer 座位几乎一生都处于禁用状态，因为一旦跑过一个轮次，preset 即固定。它移到了新建会话界面、工作区选择器旁边，选择在那里是**暂存**的：该界面先于它要应用到的会话存在，暂存值在某个会话成为当前会话且仍为空白时落地——这既覆盖工作区连接新建的会话，也覆盖它复用的那个空白会话，而搭 `sessions.create` 的便车会漏掉后者。它一经使用即被清空，与旁边的工作区选择器一致。至于运行中的会话在跑什么，则是其标题旁的一个只读标签：在那里放控件，等于承诺一次宿主会断然拒绝的切换。
+
 ## 考虑过的替代方案
 
 **在 scope 注册表中新增 preset 分层。** `ScopedLayers.merge()` 把全局层与恰好一个精确 scope 层合并。新增中间层可以让多个会话共用一份已挂载的组装，但它要改动 `dsh-scope` 及每个 scope 感知的注册表，换来的只是毫秒级的开销节省，而且会让 preset 的注册获得一个没有任何 agent 拥有的生命周期。