Remove the obsolete graphics plan

Delete docs/plan-graphics-v0.6.md and update README, architecture, asset
tooling, Blender docs and MEMORY references. Completed work stays summarized
in ADR-0013, the asset-pipeline architecture and the changelog; the v1.0.0
roadmap owns future graphics work.

docs/asset-tooling.md

@@ -86,7 +86,8 @@ character authored in Blender ([ADR-0013](adr/0013-blender-authored-npc-characte
 per-slot material maps. `NpcAvatar` instantiates that skinned model, tints its
 five material slots from the seed-driven outfit, and drives the bones from the
 existing animation code; the procedural rig remains the offline fallback (see
-the [graphics plan](plan-graphics-v0.6.md)).
+[ADR-0013](adr/0013-blender-authored-npc-characters.md) and
+[`blender-assets.md`](blender-assets.md)).
 
 ```bash
 # Print the frozen contract (slots, bone names, triangle budget)