bug(exec): sandbox exec fails with exit 74 after ~30 s when a background child keeps stdout open #4641

Problem

If a command run with openshell sandbox exec leaves a background child holding stdout or stderr, the exec waits about 30 s and then fails with exit 74, "exec output could not be delivered intact", although the command itself succeeded. Busybox wget over HTTPS hits this, because its TLS helper ssl_client outlives wget.

Reproduce (v0.1.4)

openshell sandbox exec mybox --no-tty -- sh -c 'sleep 60 & echo started'
echo $?   # 74, after ~30 s

Workaround

Detach the child's stdio: sh -c 'sleep 60 </dev/null >/dev/null 2>&1 & echo started'.

Proposal

Return when the main command exits, and report output truncation separately from command failure.