log: bridge dolt's remotesrv logger into slog

remotesrv takes a *logrus.Entry and nothing else, so passing nil left it
writing through logrus' standard logger: its own format, its own stream,
and no mask between a field named token and the journal. It is the half
of this process that serves clones and pushes — the likeliest place for
a credential to reach a log field, and the one that was logging around
everything the previous commit configured.

internal/logrusbridge hands it an entry whose only exit is a
logrus.Hook: output to io.Discard, a formatter that produces nothing,
and the logrus level left wide open so the slog handler does the
filtering from the one setting in config.ini. Fields cross as attributes
rather than a formatted blob, which is what lets a mask keyed on the
attribute path fire at all.

Verified against logrus v1.9.3 rather than assumed: Entry.log fires
hooks before it writes, before Logger.Exit and before the panic, so a
Fatal or Panic record reaches slog before the process ends. Both arms
are tested.

The package takes nothing from this service and belongs beside scribe in
auxilia; it is here because it was needed here first.