feat(assets): add 3D-to-2D sprite render pipeline and /render-sprite skill

Godot @tool render scene that takes a textured 3D model and produces
2D sprites at 1024/256/64 from 4 cardinal directions with outlines
applied at working resolution. Camera at "the angle" (-72.5deg ortho).
Includes two POC wall models with Nano Banana textures, CLI wrapper
script with headless import step, and .uid files added to gitignore.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -7,6 +7,8 @@ client/.godot/
 client/export/
 client/reports/
 client/*.import
+client/**/*.uid
+client/tooling/sprite_renderer/output/*.png
 
 # Database (shared across worktrees at ../settledreach.db, not tracked)
 db/commonwealth.db*