#finalizeFile marked the sink finalized then ran the capped-artifact tail
replay before closing. A write error during that replay threw before
sink.end(), and because #finalized was already set the executor finally
paths calling dispose() could not retry the close, leaking the descriptor
and masking the original tool error.
Moved sink.end() into a finally around the tail replay and swallowed both
the replay and close errors so the descriptor is always released and
dispose() never throws.
OutputSink.dump() was the only path that closed the spill Bun.FileSink.
The bash and Python executors re-throw on failure and their finally
blocks never closed the sink, so any large-output command that errored
leaked the artifact descriptor until an unrelated read (e.g. a SKILL.md
load) hit EMFILE.
Added an idempotent OutputSink.dispose() that closes the sink exactly
once (awaiting any in-flight sink creation, guarding post-finalize
resurrection) and wired it into every executor's finally block.
Fixes#6463
roboomp