fix: ship polkit rule so greeter user can reboot/power off (v0.8.3)

The rule that grants the greeter user authorization for
org.freedesktop.login1.{reboot,power-off} lived only in the moonarch
repo and was never installed by any PKGBUILD. On a fresh install the
reboot/shutdown buttons silently failed because greetd's greeter
session is inactive in logind and polkit denies inactive sessions by
default.

Move the rule into the moongreet source tree where it belongs and
ship it via moongreet-git.

README.md

@@ -60,6 +60,14 @@ sudo cp config/moongreet.toml /etc/moongreet/moongreet.toml
    user = "greeter"
    ```
 
+4. Install the polkit rule so the greeter user can reboot / power off:
+   ```bash
+   sudo install -Dm644 config/polkit/50-moongreet-power.rules \
+       /etc/polkit-1/rules.d/50-moongreet-power.rules
+   ```
+   Without this rule, `loginctl reboot` / `loginctl poweroff` fail because
+   greetd's greeter session is inactive in logind.
+
 ## Development
 
 ```bash