fix(ui): keep manual model override after delegated subtask completes

Synthetic subagent-completion nudges were treated as the latest user model
choice and rehydrated the agent default, while setAgent preferred the agent
pin over the session override. Skip synthetic prompts for restore, preserve
manual selection-store overrides, and prefer session agent models in setAgent.

Closes openchamber/openchamber#2404

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/ui/src/stores/useConfigStore.test.ts

@@ -522,6 +522,73 @@ describe('useConfigStore provider persistence', () => {
     expect(state.currentVariant).toBe('high');
   });
 
+  test('[issue-2404] setAgent keeps session model override over agent default model', () => {
+    // Custom agent default is model-a; user manually overrode to model-b for this session.
+    // Re-applying setAgent (e.g. after delegated subtask completion rematerializes the
+    // parent) must keep model-b rather than resetting to the agent pin.
+    const sessionId = 'ses_2404_model_override';
+    const multiModelProvider = {
+      ...provider('provider', 'model-a'),
+      models: [
+        provider('provider', 'model-a').models[0],
+        provider('provider', 'model-b').models[0],
+      ],
+    };
+    useSessionUIStore.setState({ currentSessionId: sessionId });
+    useSelectionStore.getState().saveSessionModelSelection(sessionId, 'provider', 'model-b');
+    useSelectionStore.getState().saveAgentModelForSession(sessionId, 'custom-agent', 'provider', 'model-b');
+    useConfigStore.setState({
+      activeDirectoryKey: DIRECTORY,
+      providers: [multiModelProvider],
+      agents: [testAgent('custom-agent', { model: { providerID: 'provider', modelID: 'model-a' } })],
+      currentProviderId: 'provider',
+      currentModelId: 'model-b',
+      currentAgentName: 'custom-agent',
+      selectionSource: 'manual',
+      currentVariant: undefined,
+      directoryScoped: {},
+    });
+
+    useConfigStore.getState().setAgent('custom-agent');
+
+    const state = useConfigStore.getState();
+    expect(state.currentProviderId).toBe('provider');
+    expect(state.currentModelId).toBe('model-b');
+    expect(useSelectionStore.getState().getAgentModelForSession(sessionId, 'custom-agent')).toEqual({
+      providerId: 'provider',
+      modelId: 'model-b',
+    });
+  });
+
+  test('[issue-2404] setAgent uses agent default when no session override exists', () => {
+    const sessionId = 'ses_2404_agent_default';
+    const multiModelProvider = {
+      ...provider('provider', 'model-a'),
+      models: [
+        provider('provider', 'model-a').models[0],
+        provider('provider', 'model-b').models[0],
+      ],
+    };
+    useSessionUIStore.setState({ currentSessionId: sessionId });
+    useConfigStore.setState({
+      activeDirectoryKey: DIRECTORY,
+      providers: [multiModelProvider],
+      agents: [testAgent('custom-agent', { model: { providerID: 'provider', modelID: 'model-a' } })],
+      currentProviderId: 'provider',
+      currentModelId: 'model-b',
+      currentAgentName: undefined,
+      selectionSource: 'auto',
+      currentVariant: undefined,
+      directoryScoped: {},
+    });
+
+    useConfigStore.getState().setAgent('custom-agent');
+
+    const state = useConfigStore.getState();
+    expect(state.currentProviderId).toBe('provider');
+    expect(state.currentModelId).toBe('model-a');
+  });
+
   test('loadAgents does not fetch OpenCode config directly', async () => {
     useConfigStore.setState({
       activeDirectoryKey: DIRECTORY,