logging: take the log policy from ecore

setupLogging's level parser, its os.Stderr.Stat colour probe and its three-key
mask list were one of six copies. Defaults(conf, section) resolves all of it now
and the handler stays here, which is the split that package documents.

Three things follow from taking the instance's list instead of this service's
own: the mask set gains the config-file private keys and the migration DSN that
siblings had already learned to redact, NO_COLOR is honoured, and verbosity can
be set for one run with $LOG_LEVEL or -d. [dolt.sr.ht]log-level is unchanged.

log: replace logrus with slog behind auxilia's scribe handler

Every logger field this service owned was a *logrus.Entry threaded
through a constructor, which is what logrus costs for want of a usable
default. They are slog.Default().With("component", ...) now, and the
threading is gone with them; the shared middleware's panic reports land
in the same handler, which is why the daemon sets the default before
anything that can fail.

The handler is scribe's tint handler: level from [dolt.sr.ht]log-level,
source positions, and masks keyed on the attribute path for the three
credentials this service handles — the unified-login cookie, the
Internal fernet token and the Authorization header the remotesapi reads
a PAT or a keypair JWT out of. Errors go through scribe.Err, so a culpa
error's hint reaches the operator on its own line.

logrus stays in go.mod: dolt's remotesrv.ServerArgs takes a
*logrus.Entry and nothing else. It is now confined to Config.DoltLogger,
which is the only place this service names it.

dolt-git-hook is deliberately untouched: what it writes to stderr is the
notice a pushing user reads through git, not a log.