feat: initial Moongreet greeter implementation

greetd-Greeter für Wayland mit Python + GTK4 + gtk4-layer-shell.
Enthält IPC-Protokoll, User/Session-Erkennung, Power-Actions,
komplettes UI-Layout und 36 Tests (Unit + Integration).

README.md

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+# Moongreet
+
+A greetd greeter for Wayland, built with Python + GTK4 + gtk4-layer-shell.
+Part of the Moonarch ecosystem.
+
+## Features
+
+- **greetd IPC** — Communicates via `$GREETD_SOCK` (length-prefixed JSON)
+- **User list** — Parsed from `/etc/passwd` (UID 1000–65533)
+- **Avatars** — AccountsService icons, `~/.face` fallback, default SVG
+- **Sessions** — Discovered from `/usr/share/wayland-sessions/` and `/usr/share/xsessions/`
+- **Last user** — Remembered in `/var/cache/moongreet/last-user`
+- **Power actions** — Reboot / Shutdown via `loginctl`
+- **Layer Shell** — Fullscreen via gtk4-layer-shell
+
+## Requirements
+
+- Python 3.11+
+- GTK 4, PyGObject
+- gtk4-layer-shell (for Wayland fullscreen)
+- greetd
+
+## Installation
+
+```bash
+uv pip install -e .
+```
+
+## Usage
+
+Configure greetd to use Moongreet:
+
+```ini
+[default_session]
+command = "moongreet"
+```
+
+## Development
+
+```bash
+# Run tests
+uv run pytest tests/ -v
+
+# Type checking
+uv run pyright src/
+```
+
+## License
+
+MIT