Collapse mvp-system's public surface to three binary entrypoints and make every domain module private, deleting dead provider/worker/membership implementations and inlining provider config.
- lib.rs: expose only run_chat_from_args/run_orchestrator_from_args/run_worker_node_from_env (plus a crate-private in-process helper) and the cached-model consts, and demote chat/node/observability/orchestration/prompt/staging/transport to private mods
- orchestration/mod.rs: make app private, gate engine_builder behind cfg(test), drop docker_cluster from provider_adapters, tighten vastai to pub(super), and replace pub re-exports with pub(super) run_from_args/run_in_process_from_args
- orchestration/config.rs: inline VastAiConfig/ResolvedVastAiConfig/looks_remote_image (removing provider_adapters/vastai/config.rs) and drop the DEFAULT_PIPELINE_CACHED_MODEL_* consts (hoisted to lib.rs)
- orchestration/provider_adapters/vastai: delete the ProviderPlugin impl VastAiProviderPlugin and all client/bootstrap/config accessors; repoint call sites to crate-level #[path] mods for provisioning/node_provisioning/node_actor/gguf_shard/run_fsm/run_plan
- delete orchestration/{membership_readiness,token_endpoint,resource_inventory}, node/{boot_lifecycle,data_plane_bridge(-74)}, and the worker crate-internal modules (control/device_bridge/process_adapter) along with their guarantees tests
- chat/node: narrow node_image and worker_node_runtime to private and expose only pub(super) run_from_args / run_worker_node_from_env
Signed-off-by: Zachery Aaron Shores-Chmielewski <zacheryasc@gmail.com>