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.

tools/npc_pipeline.py

@@ -16,7 +16,7 @@ path must be absolute):
     python3 tools/npc_pipeline.py palette   # derive outfit colours -> palette.json
     python3 tools/npc_pipeline.py audit
 
-Contract (frozen in docs/plan-graphics-v0.6.md + ADR-0013):
+Contract (frozen in ADR-0013 and docs/blender-assets.md):
 
   * scene root "Rig" with a skinned mesh and an armature
   * bones: Head, HipL/R, KneeL/R, ShoulderL/R, ElbowL/R, HandL/R