[model-registry] No gear implementation crate; SDK missing per-model capability accessor

[cyberantonz]

Summary

gears/model-registry has docs/ and a model-registry-sdk crate (trait ModelRegistryClientV1 with model/provider CRUD in model-registry-sdk/src/api.rs), but:

1. No gear implementation crate — there is no running model-registry service that registers ModelRegistryClientV1 in ClientHub, so a co-located gear cannot resolve it at runtime.
2. SDK lacks a per-model capability accessor — it exposes get_tenant_model / list_tenant_models but no model_capabilities(model_id). Chat Engine's ModelRegistryClient::model_capabilities would have to fetch ModelV1 and map info.capabilities → ModelCapabilitySchema by hand.

Why this blocks usage

Chat Engine's first-party LLM plugin resolves session capabilities on on_session_created via list_models + model_capabilities. With no running gear and no capability accessor, there is no production source for capability resolution on the first-party path.

Acceptance criteria

• A model-registry implementation crate exists and registers ModelRegistryClientV1 in ClientHub.
• The SDK offers a per-model capabilities accessor (or a documented, stable mapping) that Chat Engine's ModelRegistryClient adapter can consume directly.