decouple briefing from traitor (#19668)

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

Content.Server/Roles/RoleBriefingSystem.cs

@@ -0,0 +1,25 @@
+namespace Content.Server.Roles;
+
+public sealed class RoleBriefingSystem : EntitySystem
+{
+    public override void Initialize()
+    {
+        base.Initialize();
+
+        SubscribeLocalEvent<RoleBriefingComponent, GetBriefingEvent>(OnGetBriefing);
+    }
+
+    private void OnGetBriefing(EntityUid uid, RoleBriefingComponent comp, ref GetBriefingEvent args)
+    {
+        if (args.Briefing == null)
+        {
+            // no previous briefing so just set it
+            args.Briefing = comp.Briefing;
+        }
+        else
+        {
+            // there is a previous briefing so append to it
+            args.Briefing += "\n" + comp.Briefing;
+        }
+    }
+}