Add delay-gate job for controlled check timing

[RLRabinowitz]

Summary

• Adds a delay-gate job that sleeps ~30s when the delay-checks label is present on a PR
• Makes required-check depend on delay-gate, deferring its CheckRun creation so it appears on page 2+ of GraphQL statusCheckRollup.contexts pagination
• Without the label, all jobs run immediately — no change to default behavior

Test plan

• Apply delay-checks label to a PR and confirm required-check starts ~30s after the workflow begins
• Verify that without the label, required-check starts immediately
• Combine delay-checks with fail-required-workflow to confirm both behaviors work together

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