Extend AI image tooling with variants; drop dev asset folders from git

asset_pipeline generate gains --variants N (saved as <stem>-1..N.png). .gitignore now excludes assets/models/concepts, assets/models/npc, assets/textures/npc/ai, and raw assets/textures/ai outputs; only game_ready and the game-facing NPC maps/models stay tracked.

.gitignore

@@ -43,4 +43,11 @@ user://
 .gitea/logs/
 build/
 
-.env
+.env
+
+# AI / asset-development outputs (regenerated by the tools; not shipped in the game)
+assets/models/concepts/
+assets/models/npc/
+assets/textures/npc/ai/
+assets/textures/ai/*
+!assets/textures/ai/game_ready/