fix: preserve queued artifact cleanup runs

[dnikolayev]

Summary

• retain all pending artifact cleanup workflow runs
• preserve FIFO processing for up to 100 queued runs

Verification

• focused artifact cleanup workflow contract
• workflow YAML and embedded shell syntax checks
• GitHub CI is the full validation gate

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between c2654b8 and ece58c2.

📒 Files selected for processing (2)

• .github/workflows/artifact-cleanup.yml
• internal/workflowcontract/artifact_cleanup_test.go

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