Implement fat-server single round-trip sync

[miratcan]

Context

Current sync requires multi-step choreography from the client (changes → entries → apply). This forces every client to be "smart."

Goal

Single sync method on the server:

Client → Server: { method: "sync", body: { token: 5, entries: [...local_changes...] } }
Server → Client: { entries: [...remote_changes...], new_token: 12 }

Client sends its token + local changes, server returns remote changes + new token. All conflict resolution happens server-side.

Why

• Dumb clients: curl, Shortcuts, React Native can sync with one call
• ZEN-AGNOSTIC: protocol is simple enough that any language can implement a client
• Prerequisite for peer-to-peer sync (every node runs a server)

Acceptance criteria

• New sync method on SyncServer
• Client sends token + entries, gets back entries + new_token
• Conflict resolution server-side
• Existing tests pass
• E2E test for single round-trip sync

Part of milestone: Portable Core