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/Language.Designer.cs

@@ -7,7 +7,7 @@
 // </auto-generated>
 //------------------------------------------------------------------------------
 
-namespace ChatTwo.Resources {
+namespace HellionChat.Resources {
     using System;
     
     
@@ -38,7 +38,7 @@ namespace ChatTwo.Resources {
         internal static global::System.Resources.ResourceManager ResourceManager {
             get {
                 if (object.ReferenceEquals(resourceMan, null)) {
-                    global::System.Resources.ResourceManager temp = new global::System.Resources.ResourceManager("ChatTwo.Resources.Language", typeof(Language).Assembly);
+                    global::System.Resources.ResourceManager temp = new global::System.Resources.ResourceManager("HellionChat.Resources.Language", typeof(Language).Assembly);
                     resourceMan = temp;
                 }
                 return resourceMan;