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>

compose.yaml

@@ -10,8 +10,7 @@ include:
   - links/compose.yaml
   - vault/compose.yaml
   - joplin/compose.yaml
-  - vert/compose.yaml
-  - paint/compose.yaml
+  - kit/compose.yaml
   - jelly/compose.yaml
   - drop/compose.yaml
   - restic/compose.yaml