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`.

README.md

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+# Moonarch PKGBUILDs
+
+Arch Linux package builds for the Moonarch ecosystem. Use with [paru](https://github.com/Morganamilo/paru) as a custom AUR source.
+
+## Packages
+
+| Package | Description |
+|---------|-------------|
+| `moonset-git` | Wayland session power menu |
+| `moonlock-git` | Wayland lockscreen with PAM and fingerprint support |
+| `moongreet-git` | greetd greeter for Wayland |
+
+## Setup
+
+Add to `~/.config/paru/paru.conf`:
+
+```ini
+[moonarch]
+Url = https://gitea.moonarch.de/nevaforget/moonarch-pkgbuilds
+```
+
+## Install
+
+```bash
+paru -S moonset-git moonlock-git moongreet-git
+```
+
+## Update
+
+Standard system upgrade picks up new versions automatically:
+
+```bash
+paru -Syu
+```