fix(ui): keep the selected model when switching agent modes

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

packages/ui/src/stores/useConfigStore.ts

@@ -2387,6 +2387,9 @@ export const useConfigStore = create<ConfigStore>()(
                         currentProviderId,
                         currentModelId,
                     } = get();
+                    // Captured before the first set below, which unconditionally
+                    // marks the selection as manual.
+                    const hadManualSelection = get().selectionSource === "manual";
 
                     set((state) => {
                         const directoryKey = state.activeDirectoryKey;
@@ -2508,8 +2511,7 @@ export const useConfigStore = create<ConfigStore>()(
                         // Prefer a session-level manual override for this agent over the
                         // agent's configured default. Re-applying setAgent after subtask
                         // completion / rematerialization must not clobber the override
-                        // (issue #2404). Explicit agent-picker switches still force the
-                        // agent default via ModelControls' shouldPreferAgentModel path.
+                        // (issue #2404).
                         if (currentSessionId) {
                             const existingAgentModel = useSelectionStore.getState().getAgentModelForSession(currentSessionId, agentName);
                             if (existingAgentModel && hasProviderModel(providers, existingAgentModel.providerId, existingAgentModel.modelId)) {
@@ -2538,6 +2540,14 @@ export const useConfigStore = create<ConfigStore>()(
                             }
                         }
 
+                        // The user has a live manual model selection and the target
+                        // agent configures no model of its own. Switching modes or
+                        // agents must not reset the selection to the settings default
+                        // (issue #2531) — mode switches are not model changes.
+                        if (hadManualSelection && currentProviderId && currentModelId) {
+                            return;
+                        }
+
                         // If the agent has no preferred model, use settings default.
                         if (settingsDefaultModel) {
                             const parsed = parseModelString(settingsDefaultModel);