done_summary previously only pointed at .out/.err without any visual
distinction, so a completed task with clean-looking stdout and a
nonzero exit still read as routine bookkeeping in the todo queue.
Rather than gating a flag on the exit code, key it on has_stderr - a
failed command mid-chain (cd bad-path && rm ...) can exit 0 while the
real evidence sits in stderr, so an exit-code trigger would filter out
precisely the cases where nothing looks wrong. .err's presence is
already the scarce, meaningful signal the Read() pointer is built on;
keying the flag on the same condition costs nothing on the common
quiet-success path (no stderr, no pointer, unchanged) and fires on
every case where something was written to stderr, including the ones
the exit code can't be trusted to reveal.
When stderr is present: header reads as a flag instead of neutral
bookkeeping, and the .err pointer is listed before .out so it's not
the last thing skimmed past on a long completion.
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