When a user's command backgrounds a child (`cmd &`, `setsid cmd & disown`,
etc.), the backgrounded grandchild inherits the write-end of our stdout
pipe via fork(). The old `for line in proc.stdout` drain never EOF'd
until the grandchild closed the pipe — so for a uvicorn server, the
terminal tool hung indefinitely (users reported the whole session
deadlocking when asking the agent to restart a backend).
Fix: switch _drain() to select()-based non-blocking reads and stop
draining shortly after bash exits even if the pipe hasn't EOF'd. Any
output the grandchild writes after that point goes to an orphaned pipe,
which is exactly what the user asked for when they said '&'.
Adds regression tests covering the issue's exact repro and 5 related
patterns (plain bg, setsid+disown, streaming output, high volume,
timeout, UTF-8).
0a02fbd842