Add project update: post a status update to a project #42

New in this port - Ruby has no equivalent, not even at the API-wrapper level. Linear has its own "Project Update" feature (a journal-style status post on a project, distinct from editing the project's own fields) via the projectUpdateCreate mutation. Add lc project update <PROJECT> --body "..." [--health onTrack|atRisk|offTrack], resolving PROJECT the same way issue list's --project does.