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.

scripts/verify-package-readme-model-experience.ts

@@ -47,6 +47,7 @@ const SENTENCE_MODEL_EXPERIENCE: Readonly<Record<string, SentenceContract>> = {
   'packages/bash/pwsh-local': { kind: 'indirect', reason: 'The executor backend delegates model rendering to dsh-tool-pwsh.' },
   'packages/code-runtime/code-runtime': { kind: 'indirect', reason: 'The service interface delegates model rendering to Code Mode in dsh-tools.' },
   'packages/code-runtime/code-runtime-worker': { kind: 'indirect', reason: 'The worker backend delegates model rendering to Code Mode in dsh-tools.' },
+  'packages/client/ui-agent-preset': { kind: 'indirect', reason: 'Browser-side settings row; the preset it selects owns every model-facing effect.' },
   'packages/preset/agent-presets': { kind: 'indirect', reason: 'The mount installs a preset\'s own plugins, which own every model-facing registration it makes visible.' },
   'packages/typert/registry': { kind: 'none', reason: 'Runtime type registry; consumers (cordis_inspect, wire faces, gates) own any model-visible projection of registry contents.' },
   'packages/typert/loader': { kind: 'none', reason: 'Loader integration only registers generated artifacts; consumers own any model-visible projection.' },