ci: add PR benchmark workflows

[zeitlinger]

Summary

Extract the PR benchmark workflows from #2252 into a standalone PR so they can land on main first.

Why

The benchmark job uploads a Markdown artifact with the PR comment contents, but the actual PR comment is posted by a separate workflow_run workflow. GitHub only triggers workflow_run workflows when the workflow file already exists on the default branch, so keeping the reporter in the same PR prevents it from commenting on that PR's benchmark run.

Splitting these workflows out first lets subsequent benchmark PRs use the comment-reporting flow as intended.

Changes

• add PR Benchmarks workflow triggered by the benchmark label on pull requests
• add PR Benchmark Report workflow triggered by workflow_run
• keep the benchmark summary as an uploaded artifact consumed by the reporter workflow

Testing

• mise run lint:fix