feat(FN-4153): complete Step 6 — add general ephemeral agents toggle

Fusion-Task-Id: FN-4153
Fusion-Task-Lineage: 6ec09467-c7a0-4a9d-9a09-16d5ef6f7a86

packages/dashboard/app/components/SettingsModal.tsx

@@ -1975,6 +1975,22 @@ export function SettingsModal({
               </label>
               <small>When enabled, AI-generated task specifications require manual approval before moving to Todo</small>
             </div>
+            <div className="form-group">
+              <label htmlFor="ephemeralAgentsEnabled" className="checkbox-label">
+                <input
+                  id="ephemeralAgentsEnabled"
+                  type="checkbox"
+                  checked={form.ephemeralAgentsEnabled !== false}
+                  onChange={(e) =>
+                    setForm((f) => ({ ...f, ephemeralAgentsEnabled: e.target.checked }))
+                  }
+                />
+                Use ephemeral task-worker agents
+              </label>
+              <small>
+                When enabled (default), Fusion spawns short-lived <code>executor-FN-XXXX</code> agents to run each task. When disabled, only permanent agents execute tasks and the scheduler auto-assigns work using the agent reporting chain. Tasks with no eligible permanent agent stay queued.
+              </small>
+            </div>
             <div className="form-group">
               <label htmlFor="completionDocumentationMode">Completion Documentation Automation</label>
               <select

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

@@ -184,6 +184,7 @@ const defaultSettings = {
   verificationFixRetries: 2,
   workflowRevisionForkOnScopeMismatch: true,
   recycleWorktrees: false,
+  ephemeralAgentsEnabled: true,
   executorAllowSiblingBranchRename: false,
   worktreeNaming: "random",
   includeTaskIdInCommit: true,
@@ -717,6 +718,24 @@ describe("SettingsModal", () => {
       }
     });
 
+    it("saves ephemeral agent toggle in project settings payload", async () => {
+      renderModal({ initialSection: "general" });
+      await waitForSettingsModalReady();
+
+      const ephemeralToggle = screen.getByLabelText("Use ephemeral task-worker agents") as HTMLInputElement;
+      expect(ephemeralToggle.checked).toBe(true);
+
+      await userEvent.click(ephemeralToggle);
+      await userEvent.click(screen.getByRole("button", { name: "Save" }));
+
+      await waitFor(() => {
+        expect(mockUpdateSettings).toHaveBeenCalled();
+      });
+
+      const payload = mockUpdateSettings.mock.calls[0][0] as Record<string, unknown>;
+      expect(payload.ephemeralAgentsEnabled).toBe(false);
+    });
+
     it("renders and saves GitHub tracking controls in the General section", async () => {
       renderModal({ initialSection: "general" });
       await waitForSettingsModalReady();