⚡ Bolt: Optimize ArtifactStore.gc with batched operations

[google-labs-jules]

💡 What: Replaced sequential fs.stat and fs.rm operations in ArtifactStore.gc with batched concurrent operations using Promise.all and a chunk size of 10. Added a removal queue for deletions instead of mixing state calculation and deletions.
🎯 Why: Sequential operations over many files cause significant performance bottlenecks and hit EMFILE limits. Doing deletions in the state calculation loop can lead to race conditions.
📊 Impact: Reduces garbage collection latency significantly when handling many artifact files, prevents EMFILE crashes, and avoids race conditions during space reclamation calculation.
🔬 Measurement: Can be verified by running the test suite (bun run test:full) to ensure no regressions, and observing improved artifact GC performance under load.

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PR created automatically by Jules for task 422276515347463281 started by @YoungSx

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