Consolidate four separate PR labeling workflows (`pr_labeler_file.yml`,
`pr_labeler_title.yml`, `pr_size_labeler.yml`, and the PR-handling half
of `tag-external-contributions.yml`) into a single `pr_labeler.yml`
workflow. The old workflows raced against each other — concurrent label
mutations could drop or duplicate labels depending on execution order. A
unified workflow with concurrency grouping eliminates that class of bug.
