Eliminate "Default" from thinking selector, always resolve to a real option

Like the model selector, the thinking selector now always resolves to an
actual thinking option ID instead of falling back to "Default" display text.

packages/app/src/components/agent-status-bar.test.ts

@@ -112,7 +112,7 @@ describe("resolveAgentModelSelection", () => {
 
     expect(selection.activeModelId).toBe("default");
     expect(selection.displayModel).toBe("Default (Sonnet 4.6)");
-    expect(selection.selectedThinkingId).toBeNull();
-    expect(selection.displayThinking).toBe("Default");
+    expect(selection.selectedThinkingId).toBe("low");
+    expect(selection.displayThinking).toBe("Low");
   });
 });