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.test.ts

@@ -589,6 +589,30 @@ describe('useConfigStore provider persistence', () => {
     expect(state.currentModelId).toBe('model-a');
   });
 
+  test('[issue-2531] setAgent keeps the manual model when switching to an agent without an override', () => {
+    const sessionId = 'ses_2531_mode_switch';
+    useSessionUIStore.setState({ currentSessionId: sessionId });
+    useConfigStore.setState({
+      activeDirectoryKey: DIRECTORY,
+      providers: [provider('deepseek', 'deepseek-v4-pro'), provider('kimi', 'kimi-k3')],
+      agents: [testAgent('build'), testAgent('plan')],
+      settingsDefaultModel: 'deepseek/deepseek-v4-pro',
+      currentProviderId: 'kimi',
+      currentModelId: 'kimi-k3',
+      currentAgentName: 'build',
+      selectionSource: 'manual',
+      currentVariant: undefined,
+      directoryScoped: {},
+    });
+
+    useConfigStore.getState().setAgent('plan');
+
+    const state = useConfigStore.getState();
+    expect(state.currentAgentName).toBe('plan');
+    expect(state.currentProviderId).toBe('kimi');
+    expect(state.currentModelId).toBe('kimi-k3');
+  });
+
   test('loadAgents does not fetch OpenCode config directly', async () => {
     useConfigStore.setState({
       activeDirectoryKey: DIRECTORY,