* fix: preserve settings default thinking variant when switching agents
When a user sets a default thinking variant (e.g. 'high') in settings and
switches between plan and build agents in a session, the variant was reset
to 'default' (undefined) instead of respecting the settings default.
Root cause: two code paths failed to fall back to settingsDefaultVariant:
1. ModelControls variant sync effect: when no per-session+agent+model
variant was saved, the effect set currentVariant to undefined instead
of falling back to settingsDefaultVariant.
2. setAgent in useConfigStore: when the target agent had a configured
model, the variant was always passed as undefined to
applyResolvedModelSelection, ignoring both the saved per-session
variant and the settings default.
Fix both paths to resolve variants in priority order:
saved variant > settingsDefaultVariant > undefined.
* fix: preserve agent variant fallback order
* fix: apply historical session variant on restore
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Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>
