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/dashboard/app/components/SettingsModal.tsx

@@ -2027,6 +2027,23 @@ export function SettingsModal({
                 Very large tool payloads may still be clipped even when this stays enabled.
               </div>
             </div>
+            <div className="form-group">
+              <label htmlFor="persistAgentThinkingLog" className="checkbox-label">
+                <input
+                  id="persistAgentThinkingLog"
+                  type="checkbox"
+                  checked={form.persistAgentThinkingLog === true}
+                  onChange={(e) =>
+                    setForm((f) => ({ ...f, persistAgentThinkingLog: e.target.checked }))
+                  }
+                />
+                Save AI thinking/reasoning in agent logs
+              </label>
+              <div className="settings-field-help">
+                When disabled (default), internal thinking deltas are not persisted as log rows.
+                Assistant text output and tool timeline entries are unchanged.
+              </div>
+            </div>
             <div className="form-group">
               <label htmlFor="fnBinaryCheckEnabled" className="checkbox-label">
                 <input

packages/dashboard/app/components/__tests__/SettingsModal.test.tsx

@@ -513,6 +513,29 @@ describe("SettingsModal", () => {
       expect(screen.getByRole("checkbox", { name: "Save tool output in agent logs" })).not.toBeChecked();
     });
 
+    it("defaults persistAgentThinkingLog checkbox to unchecked", async () => {
+      renderModal({ initialSection: "global-general" });
+      await waitForSettingsModalReady();
+
+      expect(screen.getByRole("checkbox", { name: "Save AI thinking/reasoning in agent logs" })).not.toBeChecked();
+    });
+
+    it("reflects persisted checked thinking-log value from global settings", async () => {
+      mockFetchSettings.mockResolvedValue({
+        ...defaultSettings,
+        persistAgentThinkingLog: true,
+      });
+      mockFetchSettingsByScope.mockResolvedValue({
+        global: { ...defaultSettings, persistAgentThinkingLog: true },
+        project: {},
+      });
+
+      renderModal({ initialSection: "global-general" });
+      await waitForSettingsModalReady();
+
+      expect(screen.getByRole("checkbox", { name: "Save AI thinking/reasoning in agent logs" })).toBeChecked();
+    });
+
     it("saves persistAgentToolOutput only via global settings payload", async () => {
       renderModal({ initialSection: "global-general" });
       await waitForSettingsModalReady();
@@ -532,6 +555,25 @@ describe("SettingsModal", () => {
       }
     });
 
+    it("saves persistAgentThinkingLog only via global settings payload", async () => {
+      renderModal({ initialSection: "global-general" });
+      await waitForSettingsModalReady();
+
+      await userEvent.click(screen.getByRole("checkbox", { name: "Save AI thinking/reasoning in agent logs" }));
+      await userEvent.click(screen.getByRole("button", { name: "Save" }));
+
+      await waitFor(() => {
+        expect(mockUpdateGlobalSettings).toHaveBeenCalled();
+      });
+
+      const globalPayload = mockUpdateGlobalSettings.mock.calls[0]?.[0] as Record<string, unknown>;
+      expect(globalPayload.persistAgentThinkingLog).toBe(true);
+      if (mockUpdateSettings.mock.calls.length > 0) {
+        const projectPayload = mockUpdateSettings.mock.calls[0]?.[0] as Record<string, unknown>;
+        expect(projectPayload.persistAgentThinkingLog).toBeUndefined();
+      }
+    });
+
     it("renders global default tracking repo control", async () => {
       renderModal({ initialSection: "global-general" });
       await waitForSettingsModalReady();

packages/dashboard/src/__tests__/routes-settings.test.ts

@@ -1136,6 +1136,23 @@ describe("PUT /settings/global", () => {
     expect(res.body.persistAgentToolOutput).toBe(false);
   });
 
+  it("accepts persistAgentThinkingLog in global updates", async () => {
+    const updatedMerged = { persistAgentThinkingLog: true };
+    (store.updateGlobalSettings as ReturnType<typeof vi.fn>).mockResolvedValue(updatedMerged);
+
+    const res = await REQUEST(
+      buildApp(),
+      "PUT",
+      "/api/settings/global",
+      JSON.stringify({ persistAgentThinkingLog: true }),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(200);
+    expect(store.updateGlobalSettings).toHaveBeenCalledWith({ persistAgentThinkingLog: true });
+    expect(res.body.persistAgentThinkingLog).toBe(true);
+  });
+
   it("returns 500 on update error", async () => {
     (store.updateGlobalSettings as ReturnType<typeof vi.fn>).mockRejectedValue(new Error("Write failed"));
 
@@ -1270,7 +1287,12 @@ describe("GET /settings/scopes", () => {
 
   it("returns settings separated by scope", async () => {
     (store.getSettingsByScope as ReturnType<typeof vi.fn>).mockResolvedValue({
-      global: { themeMode: "dark", defaultProvider: "anthropic", persistAgentToolOutput: true },
+      global: {
+        themeMode: "dark",
+        defaultProvider: "anthropic",
+        persistAgentToolOutput: true,
+        persistAgentThinkingLog: false,
+      },
       project: { maxConcurrent: 4, autoMerge: false },
     });
 
@@ -1280,9 +1302,11 @@ describe("GET /settings/scopes", () => {
     expect(res.body.global.themeMode).toBe("dark");
     expect(res.body.global.defaultProvider).toBe("anthropic");
     expect(res.body.global.persistAgentToolOutput).toBe(true);
+    expect(res.body.global.persistAgentThinkingLog).toBe(false);
     expect(res.body.project.maxConcurrent).toBe(4);
     expect(res.body.project.autoMerge).toBe(false);
     expect(res.body.project.persistAgentToolOutput).toBeUndefined();
+    expect(res.body.project.persistAgentThinkingLog).toBeUndefined();
   });
 
   it("returns exact response envelope shape with only global and project keys", async () => {