Add docs/offline-sync-plan.md — a ready-to-implement design document for
offline editing and synchronization in gitmost.
- Describes current state: Yjs/Hocuspocus + y-indexeddb for document body
(CRDT, offline-ready) vs REST-based structural data (online-only).
- Clarifies that PWA installability already exists (inherited from Docmost);
the missing piece is a service worker for offline app-shell.
- Defines two sync contours (CRDT body / outbox+LWW for REST) and a staged
plan M0..M4 with per-step files, acceptance criteria and risks.
- Includes conflict-resolution rules, pitfalls, npm deps, open questions
and an implementation checklist.
