feat: add VC-sync hint after successful migrate

Print a 5-line tip on stderr after a successful `brv migrate` run
(text mode only, only when migrated > 0 and not dry-run) pointing
users at the `brv vc status / add / commit / push` flow to sync
the new HTML topics to ByteRover cloud.

Stderr (not stdout) so the hint stays out of pipes; logToStderr
(not this.warn) because it's a tip, not a warning. JSON mode emits
no hint — machine consumers parse the envelope only.