Reorganize docs into category folders and drop the retired head-review page

Group the top-level docs into docs/project, docs/design, docs/assets and
docs/process, each with its own README index. Delete the superseded
docs/screens/head-review page and sheets. Rewrite every markdown link,
absolute docs path and tool/test reference to the new locations, fix the
pre-existing broken anchors and remove the dead Joke Shop photo-board link.
check_docs OK, Python tooling 167/167.

docs/adr/0017-quaternius-npc-base.md

@@ -16,7 +16,7 @@ The CC0 **Quaternius Ultimate Modular Characters** pack already provides a
 coherent low-poly human silhouette, 21 modular outfits and 24 animation clips.
 The researched GLBs (`Casual`, `Casual_Hoodie`, `Worker`) measure 4 meshes, 62
 joints and 24 clips each. The owner directed that Quaternius become the NPC
-base. The full evaluation is in [`docs/npc-model-sourcing.md`](../npc-model-sourcing.md).
+base. The full evaluation is in [`docs/assets/npc-model-sourcing.md`](../assets/npc-model-sourcing.md).
 
 ## Decision