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.
Migrate retired /usr/_office and /usr/_desktop trees from plugin startup into /usr/plugins/<plugin>. Update office document storage, desktop session/runtime paths, and context-scoped screenshots to use the plugin-owned state layout. Add focused tests for retired-state migration and the new path behavior.
Add the built-in _desktop plugin as the owner of Xpra/Xfce lifecycle, /desktop route installation, Desktop state, session APIs, surface registration, and the linux-desktop skill. Leave _office with explicit compatibility facades and self-update delegates so users coming from 1.10 through 1.13 keep their runtime cleanup path.
