feat(web): author agent presets from a settings page

A composition is a file, but "edit it on the filesystem" is not a browser
affordance. The roster gains `read`/`write`/`remove` beside `select`, and
the browser gains a settings section over them: the presets as rows, one
composition open in a YAML editor at a time, and per-row default, duplicate,
and delete.

All four authoring methods are loopback-pinned. A composition names the
plugins a session runs, so reading one is reconnaissance, writing one is
arbitrary capability, and selecting one can move a session onto a preset
that edits the live runtime. `agentPreset.list` deliberately stays ordinary
and now reports `authorable`, so a surface knows whether creating is
possible at all rather than offering a button whose save always fails.

Authoring starts by duplicating: a shipped preset opens read-only because
the deployment's copy is what a broken local one is compared against. Ids
are contained before they become directory names, and the text is parsed
with the loader's own schema, so a save cannot leave a file no session
could load.

Fixes a defect the real-composition test found: a preset written under the
user's home could never mount, because the loader resolves a row against the
composition's own directory and Node's `node_modules` walk from there never
reaches the installed harness. The mount now records the host base and sends
bare specifiers there, leaving relative paths resolving from the preset.

Also closes the coverage the earlier surfaces in this stack shipped without —
the General row, the composer seat, and the plugin halves now have tests.

packages/client/connection/README.zh.md

@@ -2,7 +2,7 @@
 
 [English](README.md) | 中文
 
-协议消费层：客户端插件的 apply 会挂载 `ctx.connection`（共享 API 客户端 + 当前页面的 loopback 状态 + 单消费方流循环启动器）；导出表层携带协议契约类型、`AbstractApiClient` seam，以及循环的 sink／配置类型。浏览器载体以 HTTP POST 发送 unary／respond，并为 `events.mux` 与 `events.host` 各开一条只下行的 WebSocket；进程内载体满足同一双流抽象。Loopback hostname 判定逻辑留在包内部：`/api` Host fence 与 WebSocket upgrade 会直接使用它，其他客户端插件则消费派生的 `ctx.connection.isLoopback` 状态。node 半侧的 `/api` 路由让特权方法集（`host.pickDirectory`、`host.openPath`，以及整个配置面——`settings.describe`/`openDocument`/`update`/`replace`/`mutate` 与 `credentials.describe`/`set`/`unset`；读取与原生操作也在内，因为 describe 会返回已暴露的配置、打开操作会作用于 Host 桌面，而探测任意引用会报出某条凭据来自何处）以空信任表过信任 fence，从而钉在回环——已声明的 `trustedHosts` 授权可达其余全部方法，而这些方法在真正的认证层出现之前仍只限回环本机。平台载体与 ConnectionController 循环属于包内部；apply 负责选择并驱动它们。下行边界见 [WebSocket 下行载体 Agent Note](../../../.agents/notes/implemented/architecture/2026-08-04-websocket-downlink-carrier.md)；协议契约见 api-contracts v3 §3。
+协议消费层：客户端插件的 apply 会挂载 `ctx.connection`（共享 API 客户端 + 当前页面的 loopback 状态 + 单消费方流循环启动器）；导出表层携带协议契约类型、`AbstractApiClient` seam，以及循环的 sink／配置类型。浏览器载体以 HTTP POST 发送 unary／respond，并为 `events.mux` 与 `events.host` 各开一条只下行的 WebSocket；进程内载体满足同一双流抽象。Loopback hostname 判定逻辑留在包内部：`/api` Host fence 与 WebSocket upgrade 会直接使用它，其他客户端插件则消费派生的 `ctx.connection.isLoopback` 状态。node 半侧的 `/api` 路由让特权方法集（`host.pickDirectory`、`host.openPath`，以及整个配置面——`settings.describe`/`openDocument`/`update`/`replace`/`mutate` 与 `credentials.describe`/`set`/`unset`；读取与原生操作也在内，因为 describe 会返回已暴露的配置、打开操作会作用于 Host 桌面，而探测任意引用会报出某条凭据来自何处——以及 agent preset 的创作面 `agentPreset.select`/`read`/`write`/`remove`，因为组装指明了一个会话所运行的插件，读取它是侦察，写入它是任意能力，而选择它可以把会话切到一个能编辑活动运行时的 preset 上；`agentPreset.list` 不在其中，它只携带 id 与信任级别）以空信任表过信任 fence，从而钉在回环——已声明的 `trustedHosts` 授权可达其余全部方法，而这些方法在真正的认证层出现之前仍只限回环本机。平台载体与 ConnectionController 循环属于包内部；apply 负责选择并驱动它们。下行边界见 [WebSocket 下行载体 Agent Note](../../../.agents/notes/implemented/architecture/2026-08-04-websocket-downlink-carrier.md)；协议契约见 api-contracts v3 §3。
 
 ## /api 浏览器信任栅栏