Add /background commands (when background agent is configured) (#4407)

Co-authored-by: Bryan Morgan <bryanmorgan@google.com>

packages/cli/src/services/CommandService.test.ts

@@ -96,6 +96,7 @@ describe('CommandService', () => {
     mockConfig = {
       getIdeMode: vi.fn(),
       getCheckpointingEnabled: vi.fn(),
+      getBackgroundAgentManager: vi.fn(),
     } as unknown as Mocked<Config>;
     vi.mocked(ideCommand).mockReturnValue(null);
     vi.mocked(restoreCommand).mockReturnValue(null);