feat(FN-1041): add step session executor and execution settings UI

- Implement StepSessionExecutor class with parallel wave scheduling, conflict detection, and retry logic
- Define interfaces and utility types for step session execution (StepSession, StepSessionConfig, StepResult)
- Add Execution settings section to the dashboard SettingsModal for configuring step session behavior
- Add comprehensive tests for StepSessionExecutor (1225 lines) and SettingsModal Execution section
- Add changeset for @gsxdsm/fusion package

.changeset/add-step-session-execution.md

@@ -0,0 +1,9 @@
+---
+"@gsxdsm/fusion": minor
+---
+
+Add Execution section to dashboard settings for step-by-step session control
+
+Users can now configure step execution behavior from the dashboard Settings modal:
+- Toggle "Run each step in a new session" for better isolation and error recovery
+- Set "Maximum parallel steps" (1-4) when step sessions are enabled

packages/dashboard/app/components/SettingsModal.tsx

@@ -39,6 +39,7 @@ const SETTINGS_SECTIONS = [
   { id: "appearance", label: "Appearance", scope: "global" as const },
   { id: "scheduling", label: "Scheduling", scope: "project" as const },
   { id: "worktrees", label: "Worktrees", scope: "project" as const },
+  { id: "execution", label: "Execution", scope: "project" as const },
   { id: "commands", label: "Commands", scope: "project" as const },
   { id: "merge", label: "Merge", scope: "project" as const },
   { id: "memory", label: "Memory", scope: "project" as const },
@@ -1317,6 +1318,42 @@ export function SettingsModal({
             </div>
           </>
         );
+      case "execution":
+        return (
+          <>
+            {renderScopeBanner()}
+            <h4 className="settings-section-heading">Execution</h4>
+            <div className="form-group">
+              <label htmlFor="runStepsInNewSessions" className="checkbox-label">
+                <input
+                  id="runStepsInNewSessions"
+                  type="checkbox"
+                  checked={form.runStepsInNewSessions || false}
+                  onChange={(e) =>
+                    setForm((f) => ({ ...f, runStepsInNewSessions: e.target.checked }))
+                  }
+                />
+                Run each step in a new session
+              </label>
+              <small>Run each task step in its own fresh agent session for better isolation and error recovery. Failed steps can be retried individually.</small>
+            </div>
+            <div className="form-group">
+              <label htmlFor="maxParallelSteps">Maximum parallel steps</label>
+              <input
+                id="maxParallelSteps"
+                type="number"
+                min={1}
+                max={4}
+                value={form.maxParallelSteps ?? 2}
+                onChange={(e) =>
+                  setForm((f) => ({ ...f, maxParallelSteps: Number(e.target.value) }))
+                }
+                disabled={!form.runStepsInNewSessions}
+              />
+              <small>Maximum number of steps to run in parallel when file scopes don&apos;t overlap (1-4)</small>
+            </div>
+          </>
+        );
       case "commands":
         return (
           <>

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

@@ -25,6 +25,8 @@ const defaultSettings: Settings = {
   ntfyEvents: ["in-review", "merged", "failed"],
   taskStuckTimeoutMs: undefined,
   maxStuckKills: 6,
+  runStepsInNewSessions: false,
+  maxParallelSteps: 2,
 };
 
 vi.mock("../../api", () => ({
@@ -1274,7 +1276,7 @@ describe("SettingsModal", () => {
     const sidebar = container.querySelector(".settings-sidebar");
     expect(sidebar).toBeTruthy();
     const navItems = sidebar!.querySelectorAll(".settings-nav-item");
-    expect(navItems.length).toBe(11);
+    expect(navItems.length).toBe(12);
 
     // Labels include scope emoji indicators (🌐 for global, 📁 for project)
     const labels = Array.from(navItems).map((el) => el.textContent);
@@ -1284,6 +1286,7 @@ describe("SettingsModal", () => {
       "🌐Appearance",
       "📁Scheduling",
       "📁Worktrees",
+      "📁Execution",
       "📁Commands",
       "📁Merge",
       "📁Memory",
@@ -2072,4 +2075,67 @@ describe("SettingsModal", () => {
     fireEvent.click(screen.getByText("Save"));
     await waitFor(() => expect(addToast).toHaveBeenCalledWith("Internal server error", "error"));
   });
+
+  // --- Execution section tests ---
+
+  it("shows Execution in sidebar", async () => {
+    render(<SettingsModal onClose={onClose} addToast={addToast} />);
+    await waitFor(() => expect(fetchSettings).toHaveBeenCalled());
+
+    expect(screen.getAllByText("Execution").length).toBeGreaterThanOrEqual(1);
+  });
+
+  it("shows runStepsInNewSessions checkbox and maxParallelSteps input in Execution section", async () => {
+    render(<SettingsModal onClose={onClose} addToast={addToast} />);
+    await waitFor(() => expect(fetchSettings).toHaveBeenCalled());
+
+    fireEvent.click(screen.getByText("Execution"));
+    const checkbox = screen.getByLabelText("Run each step in a new session");
+    expect(checkbox).toBeTruthy();
+    expect(checkbox.getAttribute("type")).toBe("checkbox");
+
+    const input = screen.getByLabelText("Maximum parallel steps");
+    expect(input).toBeTruthy();
+    expect(input.getAttribute("type")).toBe("number");
+  });
+
+  it("maxParallelSteps input is disabled when runStepsInNewSessions is false", async () => {
+    render(<SettingsModal onClose={onClose} addToast={addToast} />);
+    await waitFor(() => expect(fetchSettings).toHaveBeenCalled());
+
+    fireEvent.click(screen.getByText("Execution"));
+    const input = screen.getByLabelText("Maximum parallel steps") as HTMLInputElement;
+    expect(input.disabled).toBe(true);
+  });
+
+  it("toggling runStepsInNewSessions to true enables the maxParallelSteps input", async () => {
+    render(<SettingsModal onClose={onClose} addToast={addToast} />);
+    await waitFor(() => expect(fetchSettings).toHaveBeenCalled());
+
+    fireEvent.click(screen.getByText("Execution"));
+    const checkbox = screen.getByLabelText("Run each step in a new session");
+    fireEvent.click(checkbox);
+
+    const input = screen.getByLabelText("Maximum parallel steps") as HTMLInputElement;
+    expect(input.disabled).toBe(false);
+  });
+
+  it("saving with runStepsInNewSessions true includes both fields in save payload", async () => {
+    render(<SettingsModal onClose={onClose} addToast={addToast} />);
+    await waitFor(() => expect(fetchSettings).toHaveBeenCalled());
+
+    fireEvent.click(screen.getByText("Execution"));
+    const checkbox = screen.getByLabelText("Run each step in a new session");
+    fireEvent.click(checkbox);
+
+    const input = screen.getByLabelText("Maximum parallel steps") as HTMLInputElement;
+    fireEvent.change(input, { target: { value: "3" } });
+
+    fireEvent.click(screen.getByText("Save"));
+    await waitFor(() => expect(updateSettings).toHaveBeenCalledTimes(1));
+
+    const payload = (updateSettings as ReturnType<typeof vi.fn>).mock.calls[0][0];
+    expect(payload.runStepsInNewSessions).toBe(true);
+    expect(payload.maxParallelSteps).toBe(3);
+  });
 });