⚡ Bolt: Optimize cache cleanup by parallelizing bulk file deletion

[acebytes]

💡 What: Parallelized FileManager.removeItem in CacheCleaner.swift using a sliding window TaskGroup and DispatchQueue.global(qos: .userInitiated).async.
🎯 Why: Speeds up cache directory cleanup by deleting files concurrently while avoiding cooperative thread pool exhaustion caused by blocking synchronous I/O.
📊 Impact: Significantly faster deletion of large cache directories with many files.
🔬 Measurement: Verify by executing cache cleanup and monitoring disk operations and UI responsiveness.

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PR created automatically by Jules for task 14908658245340724711 started by @acebytes

[google-labs-jules]

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