Add a pluggable Daytona backend (SANDBOX_BACKEND=daytona) that runs code on
Daytona Cloud via the Apache-2.0 SDK and DAYTONA_API_KEY, conforming to the
CodeSandbox interface. Sessions reattach to a labelled cloud sandbox by id
(waking a stopped one) and are created crash-safe so a failed setup never
orphans a paid sandbox; a configurable ceiling caps concurrent sandboxes and
auto-delete is clamped on so orphans always self-reap. File transfer is
workspace-contained with a size guard. Includes mocked unit tests and an
opt-in live smoke test.
Alex
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