Create the UI backend source boundary and CoreGateway skeleton

[quinCode]

Create the UI backend source boundary and CoreGateway skeleton

Parent

#2

What to build

Create the new LLMSTP-UI backend source structure and introduce CoreGateway as the only backend module allowed to communicate with LLMSTP-core. The first implementation should consume contract metadata and capabilities through a fake core CLI in tests, proving the gateway shape before migrating full workflows.

This slice establishes the backend boundary and the gateway testing strategy without requiring the React frontend.

Acceptance criteria

• Backend source has a clear package structure for app setup, API routes, schemas, application services, CoreGateway, and persistence.
• CoreGateway exposes intention-level methods for contract discovery and capability lookup.
• Application services do not call core CLI commands directly.
• CoreGateway tests use a fake core CLI and cover successful contract discovery, unknown fields, missing optional fields, and failed CLI execution.
• Existing server startup remains usable while the rewrite branch evolves.
• No React or frontend build is required to test this slice.

Blocked by

• Establish UI-core contract metadata and capability discovery #3