Opt-in Collaboration and Sync

Fe Reader keeps collaboration local-first. The opt_in_collaboration_sync gate is disabled by default and does not block desktop stable launch.

The preview contract requires explicit opt-in, provider capability discovery, local-only defaults, support bundle exclusions and rollback that preserves local workspaces while purging provider tokens and sync metadata.

Cloud sync, background upload, analytics and phone-home behavior are not enabled by this track.