[henryqdineen]

What changed / motivation ?

The perf and size jobs in benchmarks.yml fail on every PR opened from a fork, so each push to a fork PR sends its author a "[facebook/stylex] PR run failed: benchmarks" email. The benchmarks run fine. The failure is the final Post comment step, which gets 403 Resource not accessible by integration. For PRs from forks, GitHub gives pull_request workflows a read-only GITHUB_TOKEN, so the step can't comment. A PR from a branch in this repo, like #1882, passes both jobs, while the PRs from my fork (#1911, #1914, #1920–#1923, #1928) all fail them.

This change:

• Skips the Post comment step when the PR comes from a fork (github.event.pull_request.head.repo.full_name == github.repository).
• Writes the comparison table to $GITHUB_STEP_SUMMARY in both jobs, so fork PRs still show results on the run's summary page.
• Runs the compare step with yarn -s … run -s so the table no longer includes yarn's banner, $ node ./compare.js …, and Done in … lines, and drops the unneeded -- from the yarn calls so the steps stop logging yarn's deprecation warning.

PRs from branches in this repo work as before. The jobs never failed on regressions, since compare.js only flags them with **!!**, so this removes no gate. A crash in the benchmarks on the PR's code still fails the job.

Linked PR/Issues

Follow-up to #1400. It added permissions: pull-requests: write, which can't fix fork PRs: GitHub doesn't grant write access to fork PRs through the permissions key.

Additional Context

Other repos use the same guard together with a step summary:

• pnpm/pnpm pacquet-micro-benchmark.yml: the same find-comment and create-or-update-comment setup, with the same if: on both comment steps
• webpack/webpack code-size.yml
• vercel/workflow bundle-size.yml

If we want the comment on fork PRs too, the safe next step is the two-workflow split that facebook/react's sizebot uses. The pull_request workflow uploads the table as an artifact, and a workflow_run workflow, which gets a write token, posts it. The PR number has to come from the workflow_run event, not the artifact. I kept this PR small, but I'm happy to do that as a follow-up. Switching to pull_request_target isn't safe here, because these jobs run yarn install and scripts from the PR's code (GitHub Security Lab, and the same note in facebook/docusaurus build-perf.yml).

This PR comes from a fork, so its own perf and size checks will show whether the change works: they should pass, and the tables should appear on the run summary.

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code

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