refactor(moonarch-git): own mpv.conf directly, drop staging+install hook

Install mpv.conf straight to /etc/mpv/mpv.conf in package(), remove
the /usr/share/moonarch/mpv/ staging and the post_install copy.
modernz.conf is no longer shipped — overrides moved into mpv.conf
via script-opts-append.

moonarch-git/moonarch.install

@@ -29,12 +29,6 @@ post_install() {
     cp /usr/share/moonarch/swaync/config.json /etc/xdg/swaync/config.json
     cp /usr/share/moonarch/swaync/style.css /etc/xdg/swaync/style.css
 
-    # Deploy mpv configs (modernz.conf owned by mpv-modernz-git, overwrite deliberately)
-    install -Dm644 /usr/share/moonarch/mpv/mpv.conf /etc/mpv/mpv.conf
-    if [ -d /etc/mpv/script-opts ]; then
-        cp /usr/share/moonarch/mpv/modernz.conf /etc/mpv/script-opts/modernz.conf
-    fi
-
     # Deploy Walker config override to skel and existing user homes.
     # Only copies if user has no config yet — never overwrites manual edits.
     install -Dm644 /usr/share/moonarch/walker-config.toml \