Broker: provider dispatch (Anthropic, local models)

[adamcohenhillel]

The broker claims provider abstraction but is OpenAI-only in practice: hardcoded OPENAI_RESPONSES_URL (:27), OpenAI-Beta: responses=v1 header (:303), load_provider_registry requires providers.openai (:601-603). The "registry" only remaps OpenAI URLs.

Fix

• Registry-driven provider dispatch: each provider entry declares url, auth_scheme (bearer/API-key/OAuth), payload_shape (openai_responses, openai_chat, anthropic_messages, ...), and headers.
• Move privacy checks that assume OpenAI payload shape (count_input_images :572) behind the payload-shape adapter.
• Emit provider-agnostic audit events.
• Ship first-party support for Anthropic + a local (Ollama-compatible) endpoint.

Ties in with the on-device ModelAdapter.java seam — new providers likely also need a new adapter on the assistant side.

Size: L