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/tasks/main.yml

@@ -0,0 +1,28 @@
+---
+- name: Greetd - install packages
+  pacman:
+    name:
+      - "{{ display_manager_package }}"
+      - "{{ display_manager_greeter }}"
+    state: present
+    update_cache: yes
+  tags: [greetd, packages]
+
+- name: Greetd - configure tuigreet to launch Hyprland
+  template:
+    src: config.toml.j2
+    dest: "{{ display_manager_config }}"
+    owner: root
+    group: root
+    mode: '0644'
+  notify: restart greetd
+  tags: [greetd, config]
+
+- name: Greetd - ensure enabled as display manager
+  file:
+    src: /usr/lib/systemd/system/greetd.service
+    dest: /etc/systemd/system/display-manager.service
+    state: link
+    force: yes
+  notify: restart greetd
+  tags: [greetd, service]