refactor(namespace): rename ChatTwo.* to HellionChat.* across all source files

81 namespace declarations and 100 using directives converted via sed,
plus two FQN-aliases (ChatTwoPartyFinderPayload in PayloadHandler.cs and
ModifierFlag in KeybindManager.cs) updated. Critical: Language.Designer.cs
and HellionStrings.Designer.cs ResourceManager string arguments updated
synchronously — these are runtime reflection lookups not caught by the
C# compiler.

Two intentional ChatTwo references remain: the legacy migration path
'ChatTwo.json' in Plugin.cs (still points to upstream Chat 2's config
file by design) and the InternalsVisibleTo declaration in
AssemblyInfo.cs (handled in the upcoming repo-folder rename task).

The local alias names 'ChatTwoPartyFinderPayload' and 'ChatTwoConflictDetector'
are preserved as local symbols; only their target namespaces and references
changed.

ChatTwo/Resources/HellionStrings.Designer.cs

@@ -8,7 +8,7 @@
 
 #nullable enable
 
-namespace ChatTwo.Resources;
+namespace HellionChat.Resources;
 
 [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
 [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute]
@@ -26,7 +26,7 @@ internal class HellionStrings
         get
         {
             if (resourceMan is null)
-                resourceMan = new global::System.Resources.ResourceManager("ChatTwo.Resources.HellionStrings", typeof(HellionStrings).Assembly);
+                resourceMan = new global::System.Resources.ResourceManager("HellionChat.Resources.HellionStrings", typeof(HellionStrings).Assembly);
             return resourceMan;
         }
     }