feat(canvas): define Atlas diagram contracts

[MCamner]

What this changes

• adds the v2.1 Atlas Canvas roadmap with explicit ownership, security, testing and rollback checkpoints
• defines versioned request and response JSON Schemas for diagram handoff
• adds representative examples for six Atlas modes plus success and error responses
• documents the proposed Atlas One → proxy → Excalidraw boundary and known API uncertainties
• adds a contract check covering valid examples and malformed payloads

Why

Atlas One needs a stable, reviewable handoff contract before runtime proxy and Excalidraw integration begins. This keeps prompt routing, execution, canvas state and MQ review responsibilities separated while giving the next implementation tasks testable inputs and outputs.

Type

• Prompt / mode / pack
• UI
• Backend (Java server)
• Docs / release hygiene
• Other

Checklist

• README, ROADMAP, VERSION and CHANGELOG stay consistent (if version-relevant)
• Prompt/docs checks pass (scripts/check-prompts.sh, scripts/check-docs.sh)
• No secrets, tokens or private paths added
• No prompt encourages unsafe execution or approval bypass

Targeted contract validation: scripts/check-atlas-canvas.sh passes all committed positive examples and negative schema cases.