chore: persist db data to host path and add agent instructions

- docker-compose: mount ~/tcms-db-data for the sqlite database
- add CLAUDE.md agent coding guidelines

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yaml

@@ -9,7 +9,7 @@ services:
       - SECRET_KEY=your_secret_key_here
       - IS_DEMO=false # set to true to seed the database
     volumes:
-      - db-data:/app/backend/database
+      - ~/tcms-db-data:/app/backend/database
       - ./backend/public/uploads:/app/backend/public/uploads
 
 volumes: