⚡ Bolt: Parallelize bulk cache deletion using TaskGroup sliding window

[acebytes]

💡 What: Refactored removeContents to use withThrowingTaskGroup with a sliding window approach (maxConcurrency = 8) and made it a nonisolated async throws method with an injected FileManager.
🎯 Why: Previous implementation deleted directory contents sequentially on the actor's executor, causing slow, serialized disk I/O when cleaning caches with thousands of files. The TaskGroup securely parallelizes file deletion and retains original fail-fast semantics without legacy GCD or custom locks.
📊 Impact: Substantially speeds up cache deletion by parallelizing FileManager.removeItem calls safely within Swift's structured concurrency model.
🔬 Measurement: Observe faster cleanup times for large cache directories (e.g., Xcode DerivedData) in the application.

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PR created automatically by Jules for task 9515336209934025715 started by @acebytes

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