BACKEND_STORAGE_DRIVER=s3 swaps LocalStore for a stdlib-only SigV4
implementation — PutObject/GetObject/DeleteObject/CopyObject/ListObjectsV2
against any S3-compatible API (AWS, MinIO, Garage, R2). Storage becomes a
Store interface; storage_path records s3://bucket/key. Content-Length is
resolved by seeking or temp-file spooling — chunked uploads are rejected
by real S3 servers. Bucket auto-created on startup.
Verified live against zenko/cloudserver: bucket, put, get, copy, delete.
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
