Switching between Build and Plan modes reset the model selector to the
settings default because setAgent fell through to the settings-default
fallback whenever the target agent had no saved override, and the
explicit-switch path in ModelControls force-applied the agent's default
model, overwriting any per-agent override.
setAgent now keeps the current model selection when the user has a live
manual selection and the target agent configures no model of its own,
and the explicit-switch handler no longer clobbers saved per-agent
overrides with the agent default. Startup and pin behavior are
unchanged: the settings-default and agent-pin cascade still applies
when no manual selection exists yet.
Fixes#2531
Serhii Dziupin
264fc16f2c