Rebase conflicted pull requests in uv-dev (#20474)
Syncing `uv-dev` with upstream can leave its open pull requests
conflicted against their updated base.
Add a `uv-dev`-only workflow that identifies conflicted same-repository
pull requests and uses `codex-action` in a bounded matrix to rebase
them. Preserve the expected head revision, resolve multi-commit
conflicts with a narrowly scoped permission profile, and update each
branch with a lease-protected, repository-scoped app token so pull
request CI runs normally. Cross-repository pull requests remain visible
in the workflow summary but are skipped because the token cannot update
their heads.
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Co-authored-by: zanieb <2586601+zanieb@users.noreply.github.com>
