feat(golemcore/s3): add MinIO-backed S3 file plugin

[golemcore1]

Summary

• add a new first-party golemcore/s3 plugin module backed by the MinIO Java SDK instead of the AWS SDK
• expose an s3_files tool with full file-layer operations: list_directory, read_file, write_file, create_directory, delete, move, copy, and file_info
• support inline UTF-8 text reads, binary attachments for arbitrary object types, and base64 writes for non-text uploads
• add dashboard settings for endpoint, region, credentials, bucket, root prefix sandbox, policy flags, auto-create bucket, and Test Connection
• register the module in the reactor, publish initial registry metadata, and document the plugin in the repository README
• cover the implementation with mocked service, provider, settings, wiring, and MinIO client tests