⚡ Bolt: [performance improvement] Optimize artifact garbage collection I/O

[google-labs-jules]

💡 What: Refactored ArtifactStore.gc to use batched, concurrent operations (Promise.all with a chunk size of 10) for evaluating file stats and deleting expired artifacts.
🎯 Why: The original implementation awaited fs.stat and fs.rm sequentially inside a loop. When processing a high volume of artifacts, this creates a severe I/O bottleneck that slows down garbage collection and monopolizes the event loop. Concurrent batching processes files much faster without hitting EMFILE limits.
📊 Impact: Significantly reduces GC phase duration for projects with many artifacts by parallelizing I/O.
🔬 Measurement: Run the internal artifact store tests (bun test tests/unit/sub-agent/artifacts/ArtifactStore.test.ts). The optimized code retains exact functional equivalence while enabling the JS engine to dispatch multiple fs calls to the underlying thread pool simultaneously.

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PR created automatically by Jules for task 18314623730833482496 started by @YoungSx

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