feat(sandbox): Daytona sandbox provider #1687

Add a Gym sandbox provider backed by Daytona.

• Underlying tech: Daytona SDK
• Runtime / compute backend: Daytona cloud
• Existing work: PR Add Daytona sandbox provider #1513

Parent epic

#1685

Acceptance Criteria

• Provider class implementing SandboxProvider, registered in registry.py
• YAML config + example, selectable via sandbox_provider
• Passing tests (skip-guarded if the backend/SDK is not installed)
• README documenting setup + the isolation/security properties it gives
• Validated end-to-end on at least one runtime (rollouts collected)