Tie the decided line's write-up label to its prose job

The settled decision line said "writing up…" for every conversation
decision without prose, even after the prose job failed. Card metadata now
carries the current saved generation's prose job while it is queued,
running, or failed. The line shows "Writing up…" only while work is
pending, "Couldn't write this up" with Retry (the existing job retry) on
failure, and nothing once the job finishes. Chat's collapsed decision
line now names the owner as @handle like the document.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>