A nested repository is stored as a gitlink, so its files are not in the
checkpoint. Say so when the checkpoint is taken: its message gains
"[nested git repos not captured: <paths>]", which every /rollback listing
(CLI, gateway, TUI, Desktop) already shows. The gitlink parse is shared
with the rollback guard.
rollback.list sent the TUI an always-empty message (it read a "message"
key the manager calls "reason"), so TUI and Desktop listings never showed
what a checkpoint was; map it from reason.
Co-authored-by: FroRaut <79215537+Froraut@users.noreply.github.com>
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