[WIP] Add automated workflow to cleanup branches

[Copilot]

• Align staging-aggregate branch retention to keep 5 (not 10) to match staging-aggregate.yaml
• Fix grep pattern to use exact word boundaries for protected branch matching
• Use git for-each-ref for more reliable branch listing
• Exclude staging-aggregate-* branches from stale branch cleanup
• Add Git identity configuration before git operations
• Fix stdin consumption issue in stale branch cleanup loop
• Add workflow-level concurrency control with queuing
• Add job-level timeouts to prevent blocking
• Test workflow changes

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