test: verify copy-pr-bot + E2E Gate flow (do not merge)

[pimlock]

🧪 TEST PR — DO NOT MERGE

This PR exists only to verify the new copy-pr-bot + Branch E2E Checks + E2E Gate flow landed in #922 behaves as expected end-to-end.

What to observe

1. copy-pr-bot auto-mirrors this PR to a pull-request/<N> branch once marked ready-for-review (this PR is opened as non-draft to trigger that). Commit is signed, author is a trusted org member, so no /ok to test should be required.
2. Branch E2E Checks should fire on the pull-request/<N> push. With no label applied, pr_metadata should set should_run=false and the builds + E2E matrix should be skipped.
3. E2E Gate / E2E job should pass as a no-op (label absent).
4. After labeling with test:e2e:
   • E2E Gate / E2E should flip red (label applied but Branch E2E Checks has not run successfully for this SHA).
   • Re-run the latest Branch E2E Checks run from the Actions UI. pr_metadata should now see the label, and the full build + E2E matrix should execute on build-arm64.
   • When Branch E2E Checks completes successfully, E2E Gate / E2E should auto-re-run via the workflow_run trigger and flip green without manual intervention.

Rollback

Close without merging. The single commit is a no-op trailing newline in README.md.