feat(FN-2689): standardize cross-platform shell selection

- Add engine shell utility to resolve the correct shell per platform
- Update routine runner to execute commands through shared shell selection
- Update cron runner to use the same cross-platform shell behavior
- Apply shared shell handling in dashboard routes for command execution
- Add shell utility tests covering platform-specific selection behavior

packages/dashboard/src/routes.ts

@@ -3976,11 +3976,13 @@ async function executeSingleCommand(
   const { promisify } = await import("node:util");
   const execAsyncFn = promisify(exec);
 
+  const isWindows = process.platform === "win32";
+
   try {
     const { stdout, stderr } = await execAsyncFn(command, {
       timeout: timeoutMs ?? DEFAULT_AUTOMATION_TIMEOUT_MS,
       maxBuffer: AUTOMATION_MAX_BUFFER,
-      shell: "/bin/sh",
+      shell: isWindows ? "cmd.exe" : "/bin/sh",
     });
 
     return {

packages/engine/src/__tests__/shell-utils.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it, vi, afterEach } from "vitest";
+
+describe("shell-utils", () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+    vi.resetModules();
+  });
+
+  it.each([
+    ["win32", "cmd.exe"],
+    ["linux", "/bin/sh"],
+  ])("returns %s shell as %s", async (platform, expectedShell) => {
+    vi.spyOn(process, "platform", "get").mockReturnValue(platform as NodeJS.Platform);
+
+    const { defaultShell } = await import("../shell-utils.js");
+
+    expect(defaultShell).toBe(expectedShell);
+  });
+});

packages/engine/src/cron-runner.ts

@@ -4,6 +4,7 @@ import type { TaskStore } from "@fusion/core";
 import type { AutomationStore } from "@fusion/core";
 import type { ScheduledTask, AutomationRunResult, AutomationStep, AutomationStepResult, Column, TaskCreateInput } from "@fusion/core";
 import { createLogger } from "./logger.js";
+import { defaultShell } from "./shell-utils.js";
 
 const execAsync = promisify(exec);
 const log = createLogger("cron-runner");
@@ -266,7 +267,7 @@ export class CronRunner {
       const { stdout, stderr } = await execAsync(schedule.command, {
         timeout: timeoutMs,
         maxBuffer: MAX_BUFFER,
-        shell: "/bin/sh",
+        shell: defaultShell,
       });
 
       const output = truncateOutput(stdout, stderr);
@@ -421,7 +422,7 @@ export class CronRunner {
       const { stdout, stderr } = await execAsync(step.command, {
         timeout: timeoutMs,
         maxBuffer: MAX_BUFFER,
-        shell: "/bin/sh",
+        shell: defaultShell,
       });
 
       return {

packages/engine/src/routine-runner.ts

@@ -24,6 +24,7 @@ import type {
 import type { HeartbeatMonitor } from "./agent-heartbeat.js";
 import type { AiPromptExecutor } from "./cron-runner.js";
 import { createLogger } from "./logger.js";
+import { defaultShell } from "./shell-utils.js";
 
 const log = createLogger("routine-runner");
 const execAsync = promisify(exec);
@@ -264,7 +265,7 @@ export class RoutineRunner {
       const { stdout, stderr } = await execAsync(command, {
         timeout: timeoutMs ?? DEFAULT_TIMEOUT_MS,
         maxBuffer: MAX_BUFFER,
-        shell: "/bin/sh",
+        shell: defaultShell,
       });
 
       return {

packages/engine/src/shell-utils.ts

@@ -0,0 +1 @@
+export const defaultShell = process.platform === "win32" ? "cmd.exe" : "/bin/sh";