Add conditional restart support to client, bot, and bridge roles

For each of the 34 roles (3 clients, 9 bots, 22 bridges), this commit:
- Adds `_restart_necessary: false` default variable
- Adds `register:` directives to config/image/systemd tasks
- Computes `_restart_necessary` via set_fact (OR of all .changed results)
- Wires `(_restart_necessary | bool)` in group_vars/matrix_servers

This allows the systemd service manager to skip unnecessary restarts
when running install-* tags and nothing actually changed.

Service roles and complex multi-service roles will follow separately.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

roles/custom/matrix-bridge-hookshot/defaults/main.yml

@@ -380,3 +380,13 @@ matrix_hookshot_registration_extension: "{{ matrix_hookshot_registration_extensi
 # Holds the final registration (a combination of the default and its extension).
 # You most likely don't need to touch this variable. Instead, see `matrix_hookshot_registration_yaml`.
 matrix_hookshot_registration: "{{ matrix_hookshot_registration_yaml | from_yaml | combine(matrix_hookshot_registration_extension, recursive=True) }}"
+
+# matrix_hookshot_restart_necessary controls whether the service
+# will be restarted (when true) or merely started (when false) by the
+# systemd service manager role (when conditional restart is enabled).
+#
+# This value is automatically computed during installation based on whether
+# any configuration files, the systemd service file, or the container image changed.
+# The default of `false` means "no restart needed" — appropriate when the role's
+# installation tasks haven't run (e.g., due to --tags skipping them).
+matrix_hookshot_restart_necessary: false