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/README.md

@@ -0,0 +1,35 @@
+# dsh-client-ui-agent-preset
+
+English | [中文](README.zh.md)
+
+The agent-preset surface: one General-settings row choosing which [preset](../../preset/agent-presets/README.md) new sessions are composed from.
+
+## Why it is a new-session preference
+
+A session's preset is fixed when the session is created — the host refuses to adopt an existing session under a different one, because that session's history was produced under the first preset's tools. So this row cannot be a live switch, and it says so: changing it applies to sessions started afterwards while running sessions keep the composition they began with.
+
+## What it reads and writes
+
+Options and the current default both come from one `agentPreset.list` call. The roster already reports which id a session with no explicit choice gets, so the row needs no settings-schema introspection; the write targets the `agent-presets` settings namespace's `default` field, which is what the host resolves at creation.
+
+A locally authored preset is exactly as privileged as the plugins it names, so the list marks `user` rows rather than presenting every preset as shipped and vetted.
+
+The row re-reads on `settings/changed` for its own namespace and on `connection/reset`: the roster is a live directory and the default is a settings field, so an external edit or a reconnect can both move it.
+
+## When the row is absent
+
+A deployment that composes no presets answers with an empty roster, and the row renders nothing — every session then shares the host composition, and there is nothing to choose between.
+
+## Model Experience
+
+Indirectly, through the preset a later session is composed from; [`dsh-agent-presets`](../../preset/agent-presets/README.md) owns what that composition puts in front of the model.
+
+#### KV Cache effect
+
+No direct invalidation. Changing the default never touches a running session's prefix; a session created afterwards establishes its own prefix from its own composition.
+
+## Known Limitations and Deferred Work
+
+- **No per-session choice at creation** — this row sets the default only. The wire already carries `agentPreset` on `session.create`, so a session-start surface can offer the choice; that surface does not exist yet.
+- **Presets are listed by id** — a preset carries no display metadata, so the menu shows directory names.
+- **No authoring** — creating, editing, or deleting a preset is a filesystem act; this surface only chooses among what the roster supplies.