Runner hook output is no longer lost when the hook exits

A hook is a command Claude Code runs automatically at a set point. In runner mode, the spawn-runner hook's output is read and logged, and that reading changed:

• An unfinished last line is written out when the hook exits instead of being dropped.
• Output from processes the hook left running keeps being logged as after-exit lines, up to a cap.
• Errors while reading the hook's normal output or error output are handled.
• When the hook exits, a short grace period now passes before it is treated as finished.