feat: migrate from SDDM to greetd+tuigreet

- Add greetd role with tuigreet greeter (3 packages, 4.9MB)
- Replace SDDM as display-manager.service with greetd
- Add ADR 0003 documenting migration rationale
- Rename hyprland task: "register session in SDDM" -> "register
  Wayland session desktop entry"
- Add display_manager variables to vars/main.yml
- Add greetd role to workstation.yml
- Update setup.md timestamp

ansible/roles/greetd/templates/config.toml.j2

@@ -0,0 +1,8 @@
+[terminal]
+# The VT to run the greeter on
+vt = 1
+
+# Default session: tuigreet greeter
+[default_session]
+command = "tuigreet --time --remember --asterisks --cmd {{ hyprland_cmd | default('Hyprland') }}"
+user = "greeter"