Document the settings, the server layout and the new commands

The README had no complete list of settings, so gitkey being inert and
`remotes` being undocumented were invisible. It now carries a reference
table of every setting with its meaning and whether a project .mgshrc may
override it, kept honest by a test that checks each listed key really has
an MGSH_* override.

It also documents what mgsh expects of the git server, which was assumed
but never written down: the login directory of gituser *is* gitpath —
every remote command runs there without a cd — and archive/ has to exist
before `archive` and `clone -a` can work. Plus the per-project config,
the multi-target pushremote forms, `config`, and how the project is
derived from the working directory in command-line mode.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

version.txt

@@ -1 +1 @@
-4.0.11
+4.0.23