feat: initial PKGBUILD collection for the Moonarch ecosystem

Add PKGBUILDs for moonset-git, moonlock-git, and moongreet-git.
Intended as a paru custom AUR source for easy installation of
all Moonarch components via `paru -S moonset-git`.

moongreet-git/moongreet.install

@@ -0,0 +1,17 @@
+# ABOUTME: pacman install hooks for Moongreet.
+# ABOUTME: Sets ownership on cache directory and prints setup instructions.
+
+post_install() {
+    if getent passwd greeter > /dev/null 2>&1; then
+        chown greeter:greeter /var/cache/moongreet
+        chown greeter:greeter /var/cache/moongreet/last-session
+    fi
+
+    echo "==> Moongreet installed."
+    echo "==> Add moongreet to your greeter compositor command in /etc/greetd/config.toml."
+    echo "==> Adjust wallpaper: /etc/moongreet/moongreet.toml"
+}
+
+post_upgrade() {
+    post_install
+}