⚡ Bolt: [performance improvement] Batched GC file operations

[google-labs-jules]

💡 What: Replaced sequential await loops inside ArtifactStore.gc for fs.stat and fs.rm with chunked, concurrent Promise.all execution.
🎯 Why: Iterating sequentially over large number of temporary files forces O(N) individual blocking ticks which becomes a performance bottleneck and causes unneeded latency. Batching speeds up concurrent operations without hitting EMFILE limits.
📊 Impact: Considerably faster execution of ArtifactStore GC runs, saving CPU time and avoiding main thread pauses.
🔬 Measurement: Run unit tests / integration tests, check that artifacts are removed gracefully without hanging.

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PR created automatically by Jules for task 1343462849221530974 started by @YoungSx

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