feat(web): choose the default agent preset from General settings

One settings row picking which preset new sessions are composed from.

It is deliberately a new-session preference, not a live switch: a session's
preset is fixed at creation and the host refuses to adopt an existing session
under a different one, so the row says "applies to sessions you start from now
on" rather than implying it can retune a running agent.

Options and the current value come from one `agentPreset.list` call — the
roster already reports which id an unspecified session gets, so the row needs
no settings-schema introspection, unlike the permission row it is modelled on.
The write targets only the namespace's `default` field.

The menu marks `user` rows: a locally authored preset is exactly as privileged
as the plugins it names, and presenting it identically to a shipped one would
hide that.

An empty roster reads as `unavailable` and renders nothing, because composing
no presets is a valid deployment rather than a failure — distinct from a
roster call that failed, which surfaces its message.

packages/client/ui-agent-preset/src/index.ts

@@ -0,0 +1,9 @@
+/**
+ * Agent-preset surface plugin, node half. The empty apply exists so the plugin
+ * appears in the host cordis.yml / Loader; the browser half ships the
+ * General-settings row through exports["./client"], discovered from the
+ * package.json dshClient declaration.
+ */
+
+/** Host plugin body — no host-side behavior for this surface plugin. */
+export function apply(): void {}