handleCtrlZ called process.kill(0, "SIGTSTP") unconditionally; on
Windows the runtime rejects the signal name with TypeError, which
propagated out of the TUI input dispatcher and crashed the agent
with an [Uncaught Exception]. Even on POSIX the call could fail
(sandboxes that block sending to pid=0, runtime-reserved signals),
and a thrown error after we already stopped the TUI and registered
a one-shot SIGCONT listener left the UI stranded with a leaked
handler that would fire on the next unrelated continue and try to
re-start() an already-running TUI.
Branch on process.platform first so Windows shows a status notice
and never calls process.kill, and wrap the POSIX kill in try/catch
that removes the SIGCONT listener and re-starts the TUI on failure.
Fixes#2036
can1357