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 {