- rename document_artifact to office_artifact and remove retired shims/facades
- make text_editor own Markdown saves, canvas-open intent, refresh, and stale-save protection
- keep Office artifacts Desktop-only with Office formats and update skills/tests
Add a builtin _editor plugin that owns Markdown API/WebSocket sessions, canvas and modal UI, live refresh, tabs, prompt Extras for active-context open files, inline close confirmation, and Close All handling.
Route Markdown document artifacts to Editor while keeping Office/Desktop focused on LibreOffice formats, and update Desktop/Office prompts, menus, compatibility shims, and regression coverage.
