fix: tighten resource-leak and null-deref hot spots

Three pre-existing upstream defects flagged by CodeRabbit, fixed in the
v1.0.0 standalone cut where we own the codebase:

- Ipc/ExtraChat.cs Dispose now unsubscribes all three IPC subscriptions
  (OverrideChannelGate, ChannelCommandColoursGate, ChannelNamesGate)
  instead of only the first; previously the latter two leaked their
  subscriptions on every plugin reload
- GameFunctions/Types/TellTarget.cs FromTarget guards against a zero
  IPlayerCharacter.Address before dereferencing the unsafe Character*
  cast; previously a missing/destroyed target object would crash the
  game on /tell construction
- GameFunctions/GameFunctions.cs ResolveTextCommandPlaceholderDetour
  null-checks the Hook reference before calling .Original instead of
  using the null-forgiving operator; defensive guard for teardown races

HellionChat/Ipc/ExtraChat.cs

@@ -49,6 +49,8 @@ public sealed class ExtraChat : IDisposable
     public void Dispose()
     {
         OverrideChannelGate.Unsubscribe(OnOverrideChannel);
+        ChannelCommandColoursGate.Unsubscribe(OnChannelCommandColours);
+        ChannelNamesGate.Unsubscribe(OnChannelNames);
     }
 
     private void OnOverrideChannel(OverrideInfo info)