RVBox v1 design set

This design set turns the initial proposal into an implementation contract. Read the documents in this order:

1. Architecture — scope, trust model, state machine, execution, storage, and retention decisions.
2. Agent protocol — WSS framing, sessions, replay, ordering, script transfer, heartbeat, and failure containment.
3. Control plane — rvc, Unix-socket gRPC, JSON-RPC, and query/foreground semantics.
4. Platform and operations — Unix/Windows contracts, recovery, storage safety, telemetry, and defaults.
5. Configuration contract — strict TOML loading, shell resolution, cross-field validation, and annotated server/client examples.
6. Go implementation plan — phased build order, package boundaries, storage/session/client details, tests, and release gates.

The wire authority is in ../protos/rvbox/v1: common.proto contains shared data types, agent.proto contains the client/server WebSocket envelopes, and control.proto defines the local gRPC service that the HTTP JSON-RPC adapter mirrors.