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.
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