refactor: roar extension architecture around focused runtime, integration, CLI, and publish seams

[TrevorBasinger]

Summary

This PR completes the extension-architecture refactor roadmap in roar and updates the docs to match the current codebase.

Key changes:

• Extracts host runtime resources so proxy lifecycle is no longer embedded in shared host execution/coordinator paths.
• Moves integration bootstrap wiring into roar.integrations and adds typed provider entry points for telemetry and VCS providers.
• Replaces ad hoc top-level CLI command wiring with a small command registry while preserving existing lazy-loading/help behavior.
• Adds a narrow remote-registry transport seam for publish flows, keeping GLaaS support intact without hard-wiring publish internals directly to the GLaaS client.
• Adds the extension-architecture refactor proposal doc and updates the design doc to reflect the current extension model.

Why

The codebase had already converged on a few focused extension seams rather than a generic plugin host. This PR makes those seams explicit, reduces cross-layer coupling, and leaves roar easier to maintain and extend without changing user-facing behavior.

Architecture Seams

This PR cleans up some cross pollination between modules and enforces the following clear boundaries.

   flowchart LR
     subgraph CLI["CLI"]
       Registry["command_registry.py"]
       CLIInit["cli/__init__.py"]
       Commands["top-level command specs"]
       Registry --> CLIInit
       CLIInit --> Commands
     end

     subgraph Runtime["Execution runtime"]
       Host["host_execution.execute_host_run"]
       Coordinator["RunCoordinator"]
       Resources["RuntimeResourceController"]
       Proxy["ProxyRuntimeResource"]
       Backends["backend adapters: local, ray, osmo"]
       Host --> Coordinator
       Host --> Resources
       Resources --> Proxy
       Coordinator --> Backends
     end

     subgraph Integrations["Integrations"]
       CoreBootstrap["core.bootstrap"]
       IntBootstrap["integrations.bootstrap"]
       Builtins["built-in providers"]
       Discovery["typed provider discovery"]
       Telemetry["roar.telemetry_providers"]
       VCS["roar.vcs_providers"]
       Legacy["roar.integrations legacy fallback"]
       CoreBootstrap --> IntBootstrap
       IntBootstrap --> Builtins
       IntBootstrap --> Discovery
       Discovery --> Telemetry
       Discovery --> VCS
       Discovery --> Legacy
     end

     subgraph Publish["Publish"]
       PublishRuntime["publish runtime and service"]
       Preparation["session, register, and put preparation"]
       Transport["RemoteRegistryTransport"]
       Glaas["GlaasRemoteRegistryTransport"]
       PublishRuntime --> Preparation
       Preparation --> Transport
       Transport --> Glaas
     end

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