Create base src/ package structure + contributor guide

[lornakelly]

Summary

Establish a clear src/ code/package structure for the editor to enforce separation of concerns (core vs UI vs integrations) and provide documentation that helps contributors quickly understand where code should live.

Goals

• Define a maintainable module layout that scales as MVP features are added.
• Keep React Flow–specific UI concerns isolated from workflow parsing/graph logic.
• Provide contributor-friendly documentation on “where things go” and why.

Non-Goals

• Implement full feature logic inside these packages (this is structure + placeholders).
• Decide final publishing strategy for each sub-package (can evolve later).
• Perfect architecture forever — this is a baseline that can be iterated.

Description

Create a src/ folder layout (and placeholder modules) with the following intent:

• src/core/
  Domain concepts and pure logic (e.g., workflow model types, validation result types, shared utilities). No React, no React Flow.

• src/sdk/
  Integration layer around the Typescript SDK.

• src/store/
  State management and selectors (UI-agnostic where possible). Contains state shape, actions, reducers/slices, and state helpers.

• src/graph/
  Graph representation and transformations (e.g. nodes/edges model)

• src/auto-layout/
  Layout algorithms + adapters (ELK integration later). Must not depend on React components.

• src/diagram/
  Diagram rendering layer (React components + React Flow specifics: node components, edge components, handles, viewport, etc.). Converts graph → React Flow nodes/edges.