v0.7.0 — stateless documents + app://instructions

What's new

• Stateless document model (#9). Every read/write tool now takes an explicit document_id — the implicit singleton cursor that let one document's edits overwrite another is gone. Breaking for direct MCP callers: open_document is removed; pass document_id to every document tool.
• app://instructions custom-instructions contract (#7). Brand voice is served to the platform via the bundle-side custom-instructions resource and wrapped in <app-custom-instructions> automatically. Settings panel trimmed to workspace-wide config; theme/components/assets editing moved into the app.

Full PRs: #9, #7