build: rename repository folder ChatTwo to HellionChat

Repository folder, csproj, solution and all CI/build paths now use
the consolidated HellionChat name.

- ChatTwo/ → HellionChat/ (git mv preserves history with --follow)
- ChatTwo.csproj → HellionChat.csproj
- ChatTwo.sln → HellionChat.sln; obsolete Tests project entry removed
  (private/untracked sandbox)
- AssemblyInfo.cs InternalsVisibleTo for ChatTwo.Tests removed
  (file emptied; can be repopulated when actual tests land)
- repo.json and yaml image URLs updated (ChatTwo/images/ → HellionChat/images/)
- .github/workflows/{build,codeql,release}.yml csproj paths
- .github/dependabot.yml directory path

Functional behavior unchanged.

HellionChat/ChatTwoConflictDetector.cs

@@ -0,0 +1,27 @@
+using System.Linq;
+using HellionChat.Resources;
+using Dalamud.Plugin;
+
+namespace HellionChat;
+
+internal static class ChatTwoConflictDetector
+{
+    private const string UpstreamInternalName = "ChatTwo";
+
+    public static void ThrowIfChatTwoIsLoaded(IDalamudPluginInterface pluginInterface)
+    {
+        var conflict = pluginInterface.InstalledPlugins
+            .FirstOrDefault(p =>
+                p.InternalName == UpstreamInternalName &&
+                p.IsLoaded);
+
+        if (conflict is null)
+            return;
+
+        var message = HellionStrings.ChatTwoConflictTitle + "\n\n" +
+                      HellionStrings.ChatTwoConflictBody + "\n\n" +
+                      HellionStrings.ChatTwoConflictAction;
+
+        throw new System.InvalidOperationException(message);
+    }
+}