decouple briefing from traitor (#19668)

Co-authored-by: deltanedas <@deltanedas:kde.org>

Content.Server/Roles/RoleSystem.cs

@@ -17,7 +17,18 @@ public sealed class RoleSystem : SharedRoleSystem
 
     public string? MindGetBriefing(EntityUid? mindId)
     {
-        // TODO this should be an event
-        return CompOrNull<TraitorRoleComponent>(mindId)?.Briefing;
+        if (mindId == null)
+            return null;
+
+        var ev = new GetBriefingEvent();
+        RaiseLocalEvent(mindId.Value, ref ev);
+        return ev.Briefing;
     }
 }
+
+/// <summary>
+/// Event raised on the mind to get its briefing.
+/// Handlers can either replace or append to the briefing, whichever is more appropriate.
+/// </summary>
+[ByRefEvent]
+public record struct GetBriefingEvent(string? Briefing = null);