DurableFile: URLs for Agent Deliverables

Field notes

The service uses SHA-256-addressed objects, so retries of identical bytes resolve to the same URL and are not billed twice. Uploads are limited to 25 MiB, while retention defaults to 14 days and can be extended to 180 days. The API also exposes machine-readable payment options and returns actionable 4xx errors with a prescribed next call, keeping the workflow oriented toward unattended clients.