## 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`

## Summary

The `PR Benchmark Report` workflow failed to comment on
[#2252](#2252) after a
successful benchmark run ([run
28865892835](https://github.com/prometheus/client_java/actions/runs/28865892835)):

```
gh: Not Found (HTTP 404)
parse "https://api.github.com/repos/prometheus/client_java/issues/comments/{\r": net/url: invalid control character in URL
```

Root cause: `workflow_run.pull_requests` is empty when the triggering
run's head branch is not on the default branch (or the PR is from a
fork). With `PR_NUMBER` empty, the subsequent
`repos/${REPO}/issues//comments` API call 404s and the paginator emits a
malformed URL, so no comment gets posted.

## Changes

- Fall back to `repos/{owner}/{repo}/commits/{sha}/pulls` to resolve the
PR from the head SHA when `workflow_run.pull_requests` is empty.
- Fail fast with an explicit error if the PR still can't be resolved, so
future regressions don't cascade into cryptic URL-parse errors.

## Test plan

- [ ] Re-run the report workflow against run 28861406560 (or a fresh
push to #2252) and confirm a benchmark comment lands on the PR.

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

## Summary

Fix the `PR Benchmark Report` workflow for forked PR benchmark runs like
#2252, where `workflow_run.pull_requests` and `commits/{sha}/pulls` can
both be empty for the upstream repository.

## Changes

- Resolve the PR number from the uploaded benchmark artifact names
first.
- Fall back to the workflow run head repository owner and branch via
`repos/{owner}/{repo}/pulls`.
- Keep the existing commit-SHA lookup as a final fallback.
- Remove the artifact ZIP download `Accept: application/octet-stream`
header because `gh api` rejected it during local replay.

## Validation

- Replayed the resolution logic against benchmark run `28928458470` and
confirmed it resolves PR #2252 and downloads artifact `8164318312`.
- Ran `mise run lint:fix`.
- Ran `git diff --check`.