Define Artemis workspace configuration

[floitsch]

Introduce the target artemis.yaml workspace model without changing existing fleet loading yet.

• identify the format through https://toit.io/schemas/artemis/workspace/v1.json
• define reusable named server configurations
• assign file or HTTP implementations independently per backend
• let HTTP backends reference a shared server plus an interface endpoint
• resolve file backend directories relative to the workspace
• keep fleet scope and embedded-device settings out of reusable server definitions
• serialize servers and backends deterministically

This intentionally leaves fleet.json and current commands unchanged. FleetStore extraction and command wiring can follow after this shape is reviewed; migration remains deferred until the target stabilizes.

Tests cover shared server resolution, workspace-only server serialization, YAML round-tripping, relative paths, schema selection, and validation.