chore: bump version to 6.0.0-alpha.19

Bug fix:
- Fixed _bmad folder stutter with agent custom files
- Removed unnecessary backup file causing installer bloat
- Improved path handling for agent customizations

package.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://json.schemastore.org/package.json",
   "name": "bmad-method",
-  "version": "6.0.0-alpha.18",
+  "version": "6.0.0-alpha.19",
   "description": "Breakthrough Method of Agile AI-driven Development",
   "keywords": [
     "agile",