Add the Windows .cmd launcher already used by browser-use/desktop #5

Hi! I hit a small Windows setup issue with the standalone repo. The shipped CLI is the extensionless POSIX sdk/browser-harness-js script, and the setup docs only describe symlinking it on macOS/Linux. From cmd.exe / PowerShell, that does not give a normal Windows command entry point.

I noticed browser-use/desktop already carries a Windows launcher at:

app/src/main/hl/stock/browser-harness-js/sdk/browser-harness-js.cmd

and its bootstrap test explicitly checks that the .cmd exists and delegates through Git for Windows bash.exe.

I copied that launcher next to the standalone CLI (with a tiny local ERRORLEVEL propagation tweak) and it has been working well on Windows 11.

Would it make sense to include the same .cmd launcher in this repo too, so standalone installs work naturally from Windows shells/PATHEXT? If so, I would be happy to send a small focused PR.