fix(bot): make tag enforcement check name formatting

PluralKit.Bot/Proxy/ProxyService.cs

@@ -125,11 +125,17 @@ public class ProxyService
 
         if (ctx.RequireSystemTag)
         {
-            var tag = ctx.SystemGuildTag ?? ctx.SystemTag;
-            if (tag == null)
-                return "This server requires PluralKit users to have a system tag, but you do not have one set.";
             if (!ctx.TagEnabled)
-                return "This server requires PluralKit users to have a system tag, but your system tag is disabled in this server.";
+            {
+                return "This server requires PluralKit users to have a system tag, but your system tag is disabled in this server. " +
+                    "Use `pk;s servertag -enable` to enable it for this server.";
+            }
+
+            if (!ctx.HasProxyableTag())
+            {
+                return "This server requires PluralKit users to have a system tag, but you do not have one set. " +
+                    "A system tag can be set for all servers with `pk;s tag`, or for just this server with `pk;s servertag`.";
+            }
         }
 
         var guild = await _cache.GetGuild(channel.GuildId.Value);

PluralKit.Core/Database/Functions/MessageContextExt.cs

@@ -0,0 +1,18 @@
+#nullable enable
+
+namespace PluralKit.Core;
+public static class MessageContextExt
+{
+    public static bool HasProxyableTag(this MessageContext ctx)
+    {
+        var tag = ctx.SystemGuildTag ?? ctx.SystemTag;
+        if (!ctx.TagEnabled || tag == null)
+            return false;
+
+        var format = ctx.NameFormat ?? ProxyMember.DefaultFormat;
+        if (!format.Contains("{tag}"))
+            return false;
+
+        return true;
+    }
+}