feat(FN-4062): add global thinking log setting to persist agent reasoning a

Adds a global `thinkingLogEnabled` setting that gates AI thinking log persistence across the engine (executor, reviewer, merger, triage, step-session) and exposes the control in the dashboard Settings modal, with tests verifying settings parity and modal behavior.

Fusion-Task-Id: FN-4062

packages/core/src/__tests__/settings-parity.test.ts

@@ -59,9 +59,16 @@ describe("settings key parity", () => {
     expect(isGlobalSettingsKey("remoteAccess")).toBe(true);
     expect(isGlobalSettingsKey("persistAgentToolOutput")).toBe(true);
     expect(isProjectSettingsKey("persistAgentToolOutput")).toBe(false);
+    expect(isGlobalSettingsKey("persistAgentThinkingLog")).toBe(true);
+    expect(isProjectSettingsKey("persistAgentThinkingLog")).toBe(false);
+    expect(isGlobalOnlySettingsKey("persistAgentThinkingLog")).toBe(true);
     expect(isGlobalSettingsKey("researchSettings")).toBe(false);
   });
 
+  it("defaults persisted thinking logs to disabled", () => {
+    expect(DEFAULT_GLOBAL_SETTINGS.persistAgentThinkingLog).toBe(false);
+  });
+
   it("includes heartbeatMultiplier in project defaults", () => {
     expect(DEFAULT_PROJECT_SETTINGS.heartbeatMultiplier).toBe(1);
   });