Add self-update command to refresh miaur from Git

Introduce `miaur self-update` / `--self-update` that clones RepoUrl@Branch,
detects user vs system install prefix, and reinstalls in place. Configurable
via conf/env; supports --noconfirm, --repo-url, and --branch.

Co-authored-by: Frank Vitrano <WHC2006@users.noreply.github.com>

completions/miaur.zsh

@@ -14,11 +14,13 @@ _miaur() {
 		'-Bi:build and install'
 		'-Sc:clean caches'
 		'-Rns:remove a package'
+		'self-update:update miaur from Git'
 		'--aur:use the AUR only'
 		'--repo:use repositories only'
 		'--skipreview:skip PKGBUILD review'
 		'--noremovemake:keep make dependencies'
 		'--noconfirm:bypass confirmation prompts'
+		'--self-update:update miaur from Git'
 		'--bottomup:show AUR results first'
 		'--help:show help'
 	)