oven: stash build-artifact dirt before papers-build rebase
xelatex leaves tracked .log/.toc/.aux files (and sometimes intermediate
.pdfs in papers/arxiv-*/) modified after each run. Those unstaged
modifications don't need to be preserved — the auto-build only commits
system/public/papers.aesthetic.computer/ + metadata.json + BUILDLOG.md —
but they block `git pull --rebase origin main`, which silently breaks
the push step and causes the oven's local main to diverge from origin.
Add a `git checkout -- papers/` cleanup between `git commit` and
`git pull --rebase`. Failures are non-fatal; any real rebase conflict
will still surface in the next step.