feat: consolidate Vert and Paint into unified Kit stack

Created new kit.pivoine.art stack with path-based routing:
- /vert: VERT file format converter (250+ formats)
- /paint: miniPaint image editor

Changes:
- Created kit/compose.yaml with both services
- Removed Sablier scale-to-zero from Vert (no longer needed)
- Deleted old vert/ and paint/ stack directories
- Updated compose.yaml includes (removed vert, paint; added kit)
- Updated arty.yml with KIT_* environment variables
- Updated CLAUDE.md documentation with new Kit section
- Updated README.md with consolidated toolkit entry

Benefits:
- Single domain for related utilities
- Simplified service management
- Maintained HTTP Basic Auth protection
- No Sablier dependency for Vert

Access:
- File converter: https://kit.pivoine.art/vert
- Image editor: https://kit.pivoine.art/paint

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

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

arty.yml

@@ -110,16 +110,11 @@ envs:
     JOPLIN_TRAEFIK_HOST: joplin.pivoine.art
     JOPLIN_APP_PORT: 22300
     JOPLIN_DB_NAME: joplin
-    # Vert
-    VERT_TRAEFIK_ENABLED: true
-    VERT_COMPOSE_PROJECT_NAME: vert
-    VERT_IMAGE: ghcr.io/vert-sh/vert:latest
-    VERT_TRAEFIK_HOST: vert.pivoine.art
-    VERT_SABLIER_ENABLED: true
-    # Paint
-    PAINT_TRAEFIK_ENABLED: true
-    PAINT_COMPOSE_PROJECT_NAME: paint
-    PAINT_TRAEFIK_HOST: paint.pivoine.art
+    # Kit (combines Vert and Paint)
+    KIT_TRAEFIK_ENABLED: true
+    KIT_COMPOSE_PROJECT_NAME: kit
+    KIT_TRAEFIK_HOST: kit.pivoine.art
+    KIT_VERT_IMAGE: ghcr.io/vert-sh/vert:latest
     # Jellyfin
     JELLY_TRAEFIK_ENABLED: true
     JELLY_COMPOSE_PROJECT_NAME: jelly