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/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);
+  });
+});