feat: [ENG-2697] surface vc/sync/cross-agent controls in onboarding tour

Msg 1 closes with a teaser line ("Also version-controlled, cloud-syncable,
and shareable across agents — more at the end") so users know more controls
exist without diluting the local-only trust climax. Msg 3 Part 1 delivers
on it with three short bullets covering brv vc, brv vc push/pull, and
connector cross-agent sharing.

Without this, the tour leaves users thinking ByteRover is local-only with
no team or sync story — they discover the full surface area accidentally
or not at all.