Infrastructure provider integration guide and contracts

Infrastructure teams can now start with a top-to-bottom provider integration guide and use separate cache and execution references while implementing their service adapters.

The references cover content and action-result formats, the existing REAPI cache transport, command preparation, resource limits, tenant isolation, and machine ownership and cleanup. They link to current implementations and regression tests and include acceptance checks for remote reuse and failures.

The documentation distinguishes contracts available today from adapter work required for a new service. It does not introduce a new provider plug-in API or add support for another hosted service.