Move RunRetentionSweepIfDue to Phase 2 (depends on MessageManager.Store)

Smoke test in Task 6 surfaced a NullReferenceException at Plugin.cs:885 —
the retention sweep was scheduled in Phase 1 but dereferences
MessageManager.Store, which is only allocated in Phase 2 (LoadAsync).
Move the call after MessageManager init. Drop the comment that wrongly
claimed independence from Phase-2 services.

HellionChat/Plugin.cs

@@ -471,12 +471,6 @@ public sealed class Plugin : IAsyncDalamudPlugin
         Interface.UiBuilder.DisableCutsceneUiHide = true;
         Interface.UiBuilder.DisableGposeUiHide = true;
 
-        // Hellion Chat — daily retention sweep, off-thread so it never
-        // blocks plugin load. Skips itself when disabled or already ran
-        // within the past 24 hours. Pure fire-and-forget on a background
-        // thread, so it doesn't depend on Phase-2 services being live.
-        RunRetentionSweepIfDue();
-
         if (Config.ShowEmotes)
             _ = EmoteCache.LoadData(); // Fire-and-forget intentional, exceptions are caught inside
     }
@@ -518,6 +512,11 @@ public sealed class Plugin : IAsyncDalamudPlugin
 
             MessageManager = new MessageManager(this);
 
+            // Daily retention sweep, fire-and-forget. Lives in Phase 2 because
+            // it dereferences MessageManager.Store. Skips itself when disabled
+            // or when it already ran within the past 24 hours.
+            RunRetentionSweepIfDue();
+
             // Hellion Chat — Auto-Tell-Tabs service. Subscribes to the
             // MessageManager's MessageProcessed event for live tells and
             // to ClientState.Logout for the cleanup pass. Created after