⚡ Bolt: [performance improvement] Parallelize removeContents for faster bulk deletion

[acebytes]

💡 What: Modified CacheCleaner.removeContents to execute directory content deletions concurrently using withThrowingTaskGroup. It uses a sliding window (max 8) and offloads the synchronous FileManager.removeItem calls to GCD (DispatchQueue.global) using continuations.
🎯 Why: Previously, deleting directories with many files (e.g., node_modules, DerivedData) used a sequential for loop calling synchronous removeItem. This resulted in O(N) blocking disk I/O on the thread, slowing down cache evictions and potentially blocking the actor/executor.
📊 Impact: Significantly accelerates bulk deletion of large directories by parallelizing up to 8 filesystem operations simultaneously while explicitly avoiding starvation of the Swift cooperative thread pool.
🔬 Measurement: Observe a substantial reduction in total cleanup time when clearing caches with extremely high file counts, such as Xcode DerivedData or nested Node.js project dependencies.

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PR created automatically by Jules for task 3198037499954523536 started by @acebytes

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