feat(trail): assignee/reviewer/type/priority on update and create

Wire --add/remove-assignee, --add/remove-reviewer, --type, and --priority onto 'trail update' (replace-set merge via a shared mergeStringSet helper), and --type/--priority/--add-assignee onto 'trail create'. Render type, priority, and reviewers in 'trail show'.

The server rejects a body update combined with any metadata field, so runTrailUpdate now splits body and metadata into separate PATCH calls; this also fixes the interactive update path, which sent status+title+body together.