_sync_back_once defers a SIGINT that lands mid-sync, then re-delivers it once the
sync completes so the user's Ctrl+C isn't lost. It did so with
os.kill(os.getpid(), signal.SIGINT). That is not graceful on Windows: os.kill
only treats CTRL_C_EVENT(0)/CTRL_BREAK_EVENT(1) as console events; any other
value (SIGINT == 2) routes to TerminateProcess(sig), so a Ctrl+C during a
remote-backend (ssh/daytona/modal) sync-back hard-kills the whole CLI session
(exit code 2) on Windows instead of raising KeyboardInterrupt.
Use signal.raise_signal(signal.SIGINT) (3.8+), which invokes the restored
handler through C raise() on every platform. Verified on Windows: raise_signal
runs the handler (graceful) while os.kill(getpid, SIGINT) TerminateProcess-es
the process. Adds a cross-platform regression test that runs on Windows too (it
stubs the locked sync body, so unlike test_file_sync_back.py it needs no fcntl).
