Fix HF dataset import internal error and training SQLite readiness.

Wrap AssistentImportHfDataset in try/catch, batch inserts in a transaction, clearer HF rows errors for gated datasets, EnsureTrainingReady with actionable message, and friendlier training UI errors instead of generic internal error.

Co-authored-by: Cursor <cursoragent@cursor.com>

AssistentMemory.Heard.cs

@@ -50,7 +50,7 @@ public sealed partial class AssistentMemory
         }
         lock (_lock)
         {
-            EnsureOpen();
+            EnsureTrainingReady();
             if (!HasColumn("train_samples", "agent_linked"))
             {
                 return;
@@ -68,7 +68,14 @@ public sealed partial class AssistentMemory
     {
         lock (_lock)
         {
-            EnsureOpen();
+            try
+            {
+                EnsureTrainingReady();
+            }
+            catch
+            {
+                return 0;
+            }
             if (!HasColumn("train_samples", "agent_linked"))
             {
                 return 0;