Read a markdown document

The markdown behind a text/markdown artifact, at its current version — what the editor opens at /documents/:artifactId. readOnly is true for a caller who may read the session but not write it, so the editor can say so instead of failing a save. Authorization: the same gate as every session route — work_session:read (or comment to write) on the session, OR space:contribute on one of its linked spaces; a desk session only for its owner. Any denial is 404.