feat(tools): add background embedding queue for thoughts

* Implement QueueThought method in BackfillTool for embedding generation
* Update CaptureTool to utilize embedding queuer for failed embeddings
* Add EmbeddingStatus field to Thought type for tracking embedding state

internal/store/thoughts.go

@@ -58,6 +58,12 @@ func (db *DB) InsertThought(ctx context.Context, thought thoughttypes.Thought, e
 		return thoughttypes.Thought{}, fmt.Errorf("commit thought insert: %w", err)
 	}
 
+	if len(thought.Embedding) > 0 {
+		created.EmbeddingStatus = "done"
+	} else {
+		created.EmbeddingStatus = "pending"
+	}
+
 	return created, nil
 }