Add WallCraft AI MVP scaffolding and documentation

onboarding.md

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+Onboarding for WallCraft AI Contributors
+
+Overview
+- This document helps new contributors get up to speed on the WallCraft AI MVP project.
+
+Getting started
+- Prerequisites: FoundryVTT v13.0.351, node environment for building modules, access to cloud AI endpoint
+- Repository layout: modules/wallcraft-ai/ with module.json and src/
+- Key files to know:
+  - module.json: module manifest
+  - src/AIPrep.js: main module entry
+  - src/AIEndpoint.js: HTTP client for cloud AI
+  - src/PreviewLayer.js: canvas overlay logic
+  - src/SettingsPanel.js: UI to configure endpoint/token/provider
+  - src/UIFlow.js: dialog and adjustment UI
+
+Development workflow
+- Plan first: use agents.md to outline tasks
+- Implement in small, testable increments
+- Use non-destructive previews first; require explicit apply
+- Write minimal tests or manual checks for payload shaping and wall conversion
+
+Conventions
+- Naming: consistent with FoundryVTT APIs (Wall, Door, Window, Scene)
+- Data models: grid-aligned coordinates, tokens as bounding boxes
+- Logging: messages go to chat or a dedicated journal entry
+
+Testing and validation
+- Manual test plan: create a sample scene, run AI Prep, verify preview, adjust, apply, verify persistence
+- Edge cases: empty AI response, missing fields, invalid coordinates
+
+Deployment and updates
+- Document how to deploy updates to cloud endpoint if users host their own
+- Provide migration notes for API changes (backward compatibility guidance)