feat: add GET /server/config endpoint for public platform config

## Description

Add a new `GET /server/config` endpoint to expose platform-level public configuration to all clients (SDK, CLI, frontend).

## Motivation

Currently there is no unified way for clients to discover platform settings like image registry info and builder image. Each new config type would require a separate endpoint. A single `/server/config` endpoint provides an extensible container for all public configuration.

## Changes

- Add `ImageRegistryConfig` and `ServerConfig` dataclasses in `rock/config.py`
- Add `server` section in YAML config (`image_registries` list + `builder_image`)
- Add `GET /server/config` route on the proxy role
- Add `get_server_config()` method in `SandboxProxyService`
- Add unit tests

Start with the dataclasses in rock/config.py, then trace the proxy-role routes and SandboxProxyService to see where the new configuration belongs. Review the YAML configuration structure and existing unit-test patterns; the work is done when GET /server/config exposes the declared public settings and the unit tests cover the endpoint and service behavior.

Written by the indexing model from the issue text.