If `opencode serve` died after the session opened, every subsequent turn failed
with an opaque httpx connection error while the container lingered. send() now
detects the exited subprocess (returncode set) and yields a clear error chunk +
turn_end so the panel shows a real "session ended — start a new chat" message;
the idle watchdog / a human reap then tears the container down.
Renn F
a0b73cf69b