fix: move paint Dockerfile into kit directory

Moved Dockerfile from paint/ to kit/ directory and updated
build context to use local directory instead of ../paint.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

kit/compose.yaml

@@ -33,7 +33,7 @@ services:
 
   paint:
     build:
-      context: ../paint
+      context: .
       dockerfile: Dockerfile
     image: minipaint:latest
     container_name: ${KIT_COMPOSE_PROJECT_NAME}_paint