Add asset creation skill and artist agent for Stickman Studio

- Introduced a new artist agent to create and review vector, raster, clothing, prop, and animation assets based on user prompts or reference images.
- Created an asset-creation skill to manage the delegation, review, approval, and handoff processes for graphical assets and animations.
- Added project format routing documentation to guide asset creation and ensure consistency in Stickman Studio.
- Established a review workflow for asset promotion, ensuring thorough validation and user approval before finalizing assets.
- Documented the specifications for scenery, backgrounds, and vector surface finishes to enhance the child experience in the Sandbox Stage.

.gitignore

@@ -6,3 +6,4 @@ addons/
 *.tmp
 .opencode/skills/playwright-cli
 *.import
+content/review/artist/